A GEDCOM file is, in fact, just a text file with the extension GED. If the download INSISTS that you save it as a TXT file, you should be able to change the extension to GED following the download.
Following the download, if the result of your download is SomeFamily.GED.TXT, simply delete the .TXT from the name. If the file is named SomeFamily.TXT, change TXT to GED.
That should do it.
EDIT: iPad wasn't mentioned in the original post, but may be the problem. Mentioned above and below, on the iPhone, the app FileExplorer Pro allows moving files, renaming, etc. It says it's for iPad also. First, you might try FileExplorer (without the Pro) and see if it works.
If it doesn't rename iPad files, it should allow you to move the file to a PC, rename it, and move it back.