To edit these buttons you're going to need a piece of software that can open .psd files like Adobe Photoshop for example. (other programs are available).
(Assuming you have Photoshop) Once you've opened the file and selected the option to 'Keep layers' you should see the image as it is in D3f4u17s post above.
If you look through the Layers menu you should see 4 main items, the one you need to make any changes to the image is called Group 1.
Click on the little triangle next to the name Group 1 to expand it and you'll see 5 new color coded layers each of which relate to a particular colored button in the image.
Click on the little triangle next to the color you want to edit to expand the item and you'll now see 4 new layers which are for (in order) the text layer, the colored part of the button, the background effect and the main background layer of the button.
To change the text for a button simple select the appropriate text layer and using the text tool click on the text you want to change on the image and start editing.
Once you've made the necessary edits and you're ready to save the buttons to use for a project, simply use the Flatten Image option and then cut and paste each button to a separate new document and save them.
