Editing themes

This site should start you off: https://www.wpbeginner.com/

Also search this forum and you’ll find tons of answers, ask a specific question with a link to your site here if you can’t find the answer. Best of luck!

The Codex is also a great resource: https://codex.www.ads-software.com/

@hastiem:
What sort of trouble your are facing? Are you customizing themes core files after installing it?

I’m having trouble with my themes, so I’ve reverted to twentyeleven. I copied the images file from my custom theme into twentyeleven, but I’m still getting the custom banner, how do I change this?