I have already filed by 2018 return as MFS, but need to amend to MFJ. My husband has not filed yet. Do we fix MFJ on his return? Or do I need to amend my return?

Since you have already filed a return, you will need to amend that return to switch to married filing jointly. However, you will need to wait to do so until your return has been fully processed, If not, you will experience processing delays. 

Follow the directions in the TurboTax FAQ below to prepare your amended return. 

https://ttlc.intuit.com/replies/7276605

Note: You will need to mail this return to the IRS. This type of return is not eligible for e-file.