Yes, you can amend the 2017 tax return from Married Filing Separately to Married Filing Jointly.
See this TurboTax support FAQ for amending a 2017 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27372
Note - If the amended 2017 return will have a federal tax refund, the amended return must be filed on or before April 15, 2021. If filed after that date the refund will be denied.
