How do I reset my Apple ID--not the password--the ID. The old email address I use is obsolete, and it won't let me switch to my new address because it says it's already in use. But it's my ID.

Hi notedly,

If you no longer use the email that's associated with your Apple ID, you can change it by following the steps outlined here: Change your Apple ID

Here's some information from the article on what to do if you can't change it:

If you try to change your Apple ID to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later.

If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it. 

In some countries or regions, you can use your phone number as your Apple ID. But if your Apple ID is an email address, you can't change it to a phone number.

Be sure to review this article for information on what to do after you change your Apple ID: What to do after you change your Apple ID or password

Regards.