If that MFS return has been filed and accepted, you are going to have to wait for it to be fully processed and then you can amend it to switch to MFJ. You cannot switch before that, because there is already a return in the system with your SSN on it. A new return using your SSN will be rejected.
And...the 1040X you need will not be available until at least mid-March.
https://ttlc.intuit.com/questions/1899507-how-do-i-amend-a-separate-return-to-married-filing-jointly
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
