Hi Niklaskarlsson,
Thanks for using Apple Support Communities. We understand you have a couple of questions regarding your Mac and Apple ID, and we're happy to help.
Let's start with what's happening with your Apple ID. The resources below can help you reset your Apple ID password, and go over all options to get a verification code sent.
- If you forgot your Apple ID password - Apple Support
- Get a verification code and sign in with two-factor authentication - Apple Support
You mentioned that you also want upgrade your macOS. You can check if your Mac is compatible with newer macOS versions, and see how to upgrade here:
- Find out which macOS your Mac is using - Apple Support
- Update macOS on Mac - Apple Support
Lastly, are you only seeing the white line on your display when using Safari, or are you seeing that at all times when using your Mac?
Thanks.