***WARNING*** --- THIS CAN SCREW UP YOUR GAME IF YOU DON'T DO THIS RIGHT! MAKE A BACKUP COPY OF ALL OF YOU TEXTURES!!! --- ***WARNING***
Step 1: Find the File!
If you have Windows 7: Just push Start and then type "Roblox" in. When the Roblox.exe appears, right-click and select "Open File Location". Go to [Roblox File Location]>content>textures.
If you have other Windows OS's: Locate your Roblox.exe file and then go to [Roblox File Location]>content>textures.