It is an easy process.
Turn off the water to the toilet. The valve is right below the bottom corner of the tank.
Flush the toilet to drain out the water. Take off the tank lid and use a sponge, towel, etc. to mop up the last bit of water remaining in the tank.
Disconnect the water line where it goes into the tank. Put down a couple towels around the base/back of the toilet to catch water from the line when you disconnect it.
While you are looking around the back of the toilet, notice a couple bolts sticking out. Not the bolts holding the seat in place. A couple smaller ones set further back. Now look inside the tank and you should see the tops of those bolts. They will be on either side of the flapper drain.
Get a wrench to hold the nut on the bottom side of the bolt, and a screwdrive to turn them from inside the tank. Loosen these up and remove the nut/washers off the bottom. Notice what all is installed and what order they were on the bolt. Should be a flat washer and a nut.
Pull the bolt up out from inside the tank. At this point the tank will be loose so be sure to support it so it does not fall on the floor and break. Pay attention to the washers on this side also. There should be a rubber washer and large flat washer on this side as well.
Now you can remove the tank and put it asside while you remove your wallpaper and refinish the wall.
Assembly is simply the reverse of the process. Before you reinstall the tank, check the condition of the bolts and washers. If they are corroded, the washers degraded, etc. Take them with you to the hardware store and get a replacement set. Best bet is go ahead and bet the new bolts for a couple dollars it is worth it to avoid the aggravation of having to take it back appart and eventually get new parts anyway.
When you do reattach the tank, tighten the bolts slowly and evenly. A little on one side then a little on the other. You want them snug, tight enough to seal and hold the tank firmly, but not crack the porcelian.
Now reattach the water line and turn the water back on. Look for leaks around the tank bolts and where your removed and reattached the water line. Make sure it flushes, etc.
If you know the brand, usually says right between the seat and tank, go to mfgr web site and see if you can download an installation manual. That should have an exploded view(
exploded toilet) so you can see how it all fits together.