thank you again for all your help. As said at the beginning, it's great to have such an image gallery you can use and also change (themes etc.) that easily. I like it to have control over my website and its files (not the case in many easy website builders).
so I will leave it as it is, as it works fine. to repeat what I did (perhaps interesting for anyone reading this): I wanted to upgrade from 1.5.20 to 1.6. The upgrader.php did not work, perhaps because of plugins, perhaps because of some of the many adaptions I made to the theme and options. So I copied the old (working) 1.5.20 to a new directory in FTP, connected it to a new database where I imported before the old (working) database of 1.5.20. Then I copied all 1.6 files from GitHub via FTP into the new directory I created for the new Coppermine version. Then I startet update.php and everything works fine (except some FTP copy errors which were perhaps not even errors).
As I like, as mentioned, to change some things in the Coppermine gallery, I also notice which functions I don't need.
so my last question is for kind of "clean up" now: when everyhing with install or upgrade of a Coppermine gallery is fine, can I just delete the php functions in FTP I don't need? I don't want to delete a lot or things I don't know anything about, just the basic features I DON'T use like:
contact.php
addfav.php
captcha.php
ecard.php
delfav.pph
calendar.php
zipdownload.php
I don't use them and there is no UI options for them in my gallery. So is it ok to delete those file then in my FTP folder?