You can't amend a single return to nothing.
If you file a joint return with your spouse through a cpa, that will automatically cancel your single return. The IRS will bill you for the return of the refund from your single return since your income and deductions will be counted on your joint return.
Or, you can amend to married filing separately and your spouse can file separately as well. But separate filing usually has higher tax rates and lower deductions.
Or, you can amend your single return to a joint return by adding all your husband' business income, expenses and deductions to your income and deductions. Your CPA can do this if you give them a printed copy of your single return to get started. Or you may be able to do it in Turbotax but without the cpa.