Toilet handles see not only heavy use but heavy moisture as well. That's why it's not surprising for them to become rusted, broken, or otherwise damaged. Luckily, replacing a toilet handle is a fairly straightforward task.
Preparing for deinstallation.
Begin by removing the lid of the tank. Be sure to set this down somewhere soft and safe--porcelain is quite brittle, and will crack easily. Now check the inside of the tank for a model number and jot it down. This will come in handy when it comes to buying an appropriate replacement handle.
Next you're going to need to unhook the chain that links the handle to the flapper. You should see a metal clasp attached to a hole at the end of the handle. Before unhooking it, make a mental note of which hole the clasp is passing through. This will help take a lot of the guesswork out of installing the new handle.
Deinstalling the old handle.
The toilet handle is held in place by means of a threaded nut. This should be removed using a crescent wrench. But before you start torquing away, it's important to know that most toilet nuts contain left-handed threads. This means that to loosen the nut you will need rotate it clockwise--in other words, the opposite direction most people assume.
It's not uncommon for the nut to be somewhat rusty. If so, it's a good idea to give it a squirt of penetrating lubricant. Not only will this make removal easier, but it will also reduce the risk of the wrench slipping and cracking the tank.
Purchasing and installing the new handle.
Be aware that not all toilet handles--or all toilets--are created equal. It is important to consider the width of the hole in the tank, in other words, the hole that the handle slots through. Otherwise, you might end up with a handle too large to fit through. If you've got a pair of calipers lying around, it's a smart idea to measure the width of either the hole or the threaded stem coming out the back of the handle.
Once you've got an appropriately sized replacement handle, all that is left is to reverse the steps above. Don't forget to tighten the nut by turning it in a counterclockwise direction. Begin tightening with your fingers, only using the wrench to firm it up at the end. Then reattach the chain to the correct hole. Congratulations, you did it! For further assistance, contact a local plumber.