If you want to change the email address linked to your ApplyWeb account, but are unable to do so because you're seeing an error message about the address already being in use, then there's an issue with the email address you're trying to add to your account.
In the screenshot above, "ExampleofAddress@collegenet.com" is the email which is already linked to an account in our system.
If the email is already in use, then that means you already have an ApplyWeb account with that address. It may be an old account or an account you accidentally created in an effort to change your email, but regardless, you will need to delete that second account in order to free up the email address.
For more information on how to delete an account, check out this article
Once that second account has been deleted, you'll then be able to update your main account with the email address.