Can you reset the nether?

To answer your question, you can reset the nether, but the new one that will generate will be the same as the old one, except without any changes you may have made. (This could be used to cheat, go to the nether, gather resources, reset the nether, gather the same resources in the same location, and so on…) To reset the nether, go to

C:\Users\<your windows username>\AppData\Roaming\.minecraft\saves\New World

on Windows or

~/Library/Application Support/minecraft

on OS X. Next go to the "/saves" folder, and then into the folder that corresponds with the world in which you want to reset the nether. Usually the folder will have the same name as the world it corresponds with. Finally go to the "/DIM-1" folder and delete anything inside. Next time you enter the nether, minecraft will reset the nether to how it was before you made any changes to it.

Note: This answers the question "how to reset the nether", but this just erases any changes you may have made to it, thus "reset". You can't really use this to find nether fortresses.

You can't reset the nether, but there are some tricks for finding Nether fortresses.

From the Minecraft Wiki article for Nether Fortresses:

Nether fortress tend to cluster together in strips that run north and south. When you're searching for your first nether fortress, traveling east or west will give you the best chances of running into one. If you travel north or south, you could travel a long distance between two strips of fortresses. But if you have already found a fortress, you should travel either north or south to find another, which will likely be within a few hundred blocks. Note that north and south spawning is just a tendency and you may not find additional fortresses simply by going north and south from a located fortress in every case. Be flexible in your search and note that additional fortresses may still be east or west from a located fortress.

You can see which direction you're traveling by pressing F3.