How do I unlink someone who , unknowingly to me, linked his phone to mine and can read my texts

No one can link their phone to yours, unless you provided them with your iCloud ID and password, then cannot read your texts either. Two phones cannot be programmed to the same Phone number to receive SMS, and iMessage is based on your Apple ID.

What makes you believe this phone is "linked"?

You’re told it’s happening, but is there evidence of this actually happening?

Wouldn’t be the first time that somebody lied, after all.

That written, here’s some Apple-provided reading material for you:

… https://manuals.info.apple.com/MANUALS/1000/MA1976/en_US/device-and-data-access-when-personal-safety-is-at-risk.pdf

I wouldn’t change Apple ID, but I would clean up passwords and security more generally, and would get two-factor authentication enabled if not already, verify the Apple ID trusted accounts and telephone numbers and devices, and change the PIN on your cellular provider account.

You can change your Apple ID by just change the Email not the account

About your Apple ID email addresses - Apple Support

If you were sharing an Apple ID, then allowed for Text Message forwarding, that may have been a possibility, however, changing your Apple ID password would have fixed that, unless you were using a 3rd party messaging app. Changing your Apple is going to cause you more trouble due to any and all content that you have downloaded to your device is locked to your former Apple ID.