It's fairly simple but it may seem overwhelming. There is 5 only things that's screwed or bolted to the tank including the handle.
You can buy a kit that has all new parts for the toilet or a universal set.
First cut the water off at the wall that leads to the tank.
Flush the toilet and it will drain most of the water out of the tank. You can get more water out with a cup or rag.
Find the hose that is attached to the waterline and unscrew it from the bottom of the tank. That is attached to the bottom of the fill valve. Undo that and take the fill valve out and pull the fill tube off of it.
There is 2 bolts that hold the tank to the toilet. Undo those and you can take the tank off.
From there, you can replace the flapper. It is screwed onto the bottom of the tank. That'll get the overflow tube also.
There's a nut on the inside of the tank that holds the handle on. Just loosen it, and you can get the handle off.
The chain should just unhook from the handle or flapper.
Now that you've got it disassembled, Clean the tank if you'd like and replace everything with the parts kit that you bought.
Everything will go in the same way that it came off.
Don't forget the washers and make sure the bolts are tight but not too tight that you might crack the porcelain.
Let me know if you have any questions about putting the new parts in or if you have a question on any of the steps I listed above.