I have already filed my tax return and I've gotten most of it back ready. My problem is that I just got my tax form from my other job. How can I file it?

You cannot change or add anything on the return that you e-filed.  It is too late, just like when you put an envelope in a US mailbox

Now you have to wait until the IRS either rejects or accepts your return. If your return is rejected, you will be able to go into your account and add or change it and resend it.

If the IRS accepts your return, then you have to wait until it has been fully processed and you have received your refund.  THEN you can prepare an amended tax return 1040X and mail it in. You cannot e-file an amended return.  Meanwhile, DO NOT change anything on your return until you start the amended return.  If you did change or add the missing info then take it out and put your return back to the way it was when you first filed it.

How to amend an Online return
https://ttlc.intuit.com/questions/4667226-how-do-i-amend-my-2018-turbotax-online-return