If you find yourself wishing you had a smaller, easier-to-turn gear to take the sting out of riding uphill, there are a few modifications you may make to your bike.
If you follow our instructions, you'll soon be spinning up the climbs, and you could even come to enjoy them!
For this post, we'll assume you have a bike with derailleur gears rather than hub gears.

The fundamentals of bicycle equipment

If you have a normal derailleur set up on a road bike, chances are you have two (perhaps one or three) chainrings on the chainset (at the front) and somewhere between eight and twelve sprockets on the rear wheel, in what's commonly referred to as a cassette or cluster.
The chainring and sprocket combination you use determines the size of the gear. Running the chain through the larger chainring produces a greater (harder) gear that propels you ahead with each pedal revolution, as does running the chain through the smaller sprocket.
But what if you're using the smallest chainring and the largest sprocket and the gear is still too hard? Of course, you can improve your fitness, but you may also make improvements to your equipment.

Cassette replacement

Cassettes are available with a variety of sprocket combinations. As an example, consider Shimano's 105 R7000 groupset. Shimano produces 105 cassettes with 11-tooth sprockets up to 28-tooth sprockets, which are typically printed as 11-28T. It also includes a 12-25T tape, an 11-30T cassette, a wide-range 11-32T cassette, and an even broader 11-34T cassette.
All else being equal, the 34T sprocket on the 11-34T cassette will provide you with the easiest gear. If your bike currently has an 11-28T cassette, upgrading to an 11-34T cassette will make climbing easier.
Swapping one cassette for another is a simple task, but it does necessitate the use of specific tools: a cassette tool and a chain whip. If you do not have these, you can have a bike shop do it for you.
Capabilities of the rear derailleur
When changing your cassette, keep in mind the maximum sprocket capacity and total capacity of your rear derailleur. This is the maximum range of chainring and sprocket sizes that it can support.
Assume you had 50-tooth and 34-tooth chainrings: the difference between the two (50 minus 34) is 16 teeth. Assume your bike is equipped with an 11-28T cassette: the difference between the large and small cassettes (28 minus 11) is 17 teeth. When you add them up, you get a total difference of 33 teeth (16 plus 17).
Shimano's short cage 105 R7000 rear derailleur accepts sprockets up to 30T and has a total capacity of 35 teeth. That is, you can utilise it with the above-mentioned arrangement. But, for two reasons, you can't utilise that rear derailleur with the same chainset and an 11-34T cassette. For starters, it will have difficulty getting onto the 34T sprocket, while it may work if your gear hanger is long enough.
Even if the derailleur can shift to the big sprocket, the overall difference you're asking it to handle is 39 teeth, thus the chain will be too slack in the combination of the smallest chainring and smallest sprocket, or too tight in the largest. This can cause transmission jamming or even break

Changing out your chainset

If you're riding a road bike, you'll most likely have a double chainset with two chainrings. There are three popular double chainring combinations: a 52 or 53-tooth outer chainring with a 39-tooth inner chainring; a 50/34T pairing, sometimes known as a compact or semi-compact 52/36T; and a 52/36T pairing.

If you have a 53/39T configuration and find the gearing too difficult, you can upgrade to a 50/34T chainset or attach smaller chainrings to your existing cranks (check that the new chainrings have the same bolt circle diameter, BCD, as the old ones).
You'll also need to adjust the location of your front mech and purchase a new chain (or at least shorten your existing chain).
Front derailleurs, like rear derailleurs, have a maximum capacity. It refers to the size difference between the large and small chainrings in this case. Assume you have 50/34T chainrings: the difference is 16 teeth.
A Shimano 105 front derailleur, for example, has a capacity of 16T, which covers all of the typical double chainset combinations.
If a compact chainset isn't small enough, consider a sub-compact like the FSA Energy. These typically have 46T and 30T chainrings, resulting in a 10% reduction in gears.

Choosing a triple chainset

Finally, switching from a double to a triple chainset is an option for adjusting your gearing. A standard road triple setup has a 30T inner chainring, but a compact setup has a 34T inner chainring.
However, this is a somewhat complex switch. You'd need to replace your shifters, chainset, and possibly your derailleurs. Rather than swapping to a triple, you're better off buying a bike that's already equipped with one.
Triple chainsets have mostly fallen out of favour for road use due to the emergence of cassettes and rear mechs with a super-wide range of gears in recent years. However, other people do better with triples; it all boils down to personal preference.
Best of luck on your climbs!