Q: How do you change your apple id?

if you created a new apple ID when you got your imac/ipad, you can continue to use that one for your old devices.

go to icloud>sign out> you will be prompted to delete icloud drive stuff,. then you will be asked to keep contacts and calendars and the important stuff. keep that

sign in with your new apple ID and merge the information

you can stay signed in to the itunes and app store in the settings so that you wont be prompted for your password everytime an app needs to be updated.

or if you want to sign out of that too and still be able to keep your purchases and your history, look into family sharing. add your other account to the group and share purchases with each other. then you can sign out of the old account and leave it be.

Family Sharing - Apple Support

Change your Apple ID - Apple Support

If you're asked for the password to your previous Apple ID in iCloud - Apple Support