Can I change filing separate status to filing jointly on 2018 tax return?

No. It’s too late. If either you or your spouse (or both of you) file a separate return, you can generally change to a joint return within 3 years from the due date (not including extensions) of the separate return or returns

Thanks for your feed back.

@senelole Sorry----it is too late to amend a 2018 return and get any difference in the refund.   You had three years to amend and change from filing separate returns to filing jointly, but you have passed the deadline for that now.

Thank you.