how do i chamge my apple id email but when i but a new email it says this email cant be used

What does 'won't let me' mean ? What happens ? If the email address is currently being used on an account, or has been used as an address recently (possibly the last 12 months) you won't be able to use it.

Changing primary email address : Change your Apple ID - Apple Support

Presumably for security reasons Apple won't let you re-use an email address which has been used for another purpose, such as a rescue email, with Apple.