Q: delete old apple id

Tthe simple answer is: sign out from the old ID, sign in with the new. You'll have to change this in multiple locations, but start with Settings > iCloud.

Having multiple Apple IDs is generally not recommended. Whatever issues you're having with the "old" ID can be addressed and modified.

https://www.apple.com/support/appleid/manage-account/

If you forgot your Apple ID - Apple Support

Change your Apple ID - Apple Support

Change your Apple ID password - Apple Support