How do i re-file an e-file i made a mistake on the amount?

If you already efiled the return and then found the error, then you will need to file an amended return.

do i have to wait for them to rejected it first ?

If it hasnt been accepted, then you can fix it and then just re efile it.  If it has been efiled and accepted, then you must amend to fix it.  An error on an amount wont necessarily cause the return to be rejected.