I agree with the above comments that the question is to broad and that you should start and ask when you hit specific problems, but here a few hints.
- You need to download the file from url.
- Open it in editor, edit it, save it...
- Upload them back to the server.
If you divide your app into sections you will get specific problems you can focus on.
So just with the above you can search for options and try to:
Download a file from internet( HTTP GET with DefaultHttpClient for example...google it ;) )
Save that file
Open it in default text reader, which will handle editing and saving.
saving the file paths in your app through SharedPreferences ( another google question ;) )
Uploading the file
and so on....
I know its hard if you dont have experience but start with the little things first, maybe just opening a file, just downloading a random file, and soon you will be able to put the pieces together like a puzzle.... good luck!