Layers in GIF

Hi, I've got a question.
I recently designed a web template, and saved it for web after having it sliced.
Now I want to edit a few of those pictures, but they're saved as .gif files, which means that it doesn't say "Background", like usual, but it just says "Index". I can't use any layer options now.
Plus, I am unabled to use any filter.
Can anyone tell me how to enable layers in .gif images and let you use filters?

Edit>Mode> and change it to RGB

But you will probably be better off going back to the Psd file, make your edits and then save the individual slices out.

Ok, thank you very much!