Is there any way that I could rename a unit without making a new side?

I'd assume that if you edit the localization strings for the unit, it will change the name of the unit ingame.

This might be a stupid question but wheres the localization string located?

Its not and I was expecting you to ask that question. However, I was hoping you won't as I don't remember the exact file of the tool or the location of the unit strings in the tool. To run the tool, there is a file like editlocalize.bat in your data_XXX folder (where XXX is the character code for your map). Double-click on the bat file and the tool will open.

The tool itself tends to crash a lot on me. It doesn't handle unexpected things very well.

Edit: Oh, and theres a whole section on the Localize tool in the modding FAQ sticky, Read that.

I found it and edited what I wanted, but where do I put it so it will change the whole game and not just a map.

You don't. The localization strings are stored in the game's main core.lvl, but we don't have all the files needed to recreate the retail version of core.lvl.

You might be able to do it by hex editing, but you'd have to ask someone else about that. Why do you want it changed for the entire game?