I use the app called "Fera" to change my cover photo (changing that photo is the only reason why I have that app). I don't know of another app that'll do this. Others have reported success with using the Safari app to log in over the Internet and changing it there (as if on a laptop or desktop computer).
If neither of those work for you, I'm afraid you'll have to find a 'regular" computer and change it with that. Sorry.
Marilyn