I figure since not alot of people can mod, and this forum has no topics, I might as well get it going.
First of all, you need San Andreas GXT Editor. I won't provide a direct link because I'm too lazy.
Once you have it, you need to press this button:
Then, you will have to choose your language. So go to:
My Computer -> Local Disc -> Program Files -> Rockstar Games -> GTA San Andreas -> Text then click on American.gxt or whatever.
Now, say you want to rename your Buffalo to BMW M3 or something. You click this button and type Buffalo
Then, on the big box on the right, it says Buffalo, change that to BMW M3 or whatever your mod is. Then click "MAIN" On the top left. Then press the save button. (Next to the open button)
More tutorials coming soon!