Gif files

Simply rename the 2 gifs you require as buy.gif and checkout.gif and import them into your /images/ folder.
These will overwrite your existing buttons and you can upload them to your server.

PNG files
These are editable files - you can modify them in Macromedia Fireworks - if you don't have the program, there's a 30 day free trial available here www.macromedia.com
Once you've edited the .pngs export them as .gif files (buy.gif and checkout.gif), save them in your /images/ folder and upload to your server.

You don't have to make any other changes.
Refresh your browser / empty your cache to check the changes.
Make sure the files you upload are .gif files.
