You may file an amended return or refund claim if the change is attributed to you and affects your tax due. If you are claiming a refund, calculate the refund amount as follows:
1) Recompute the joint tax due for your original return.
2) Recompute your and your ex-spouse’s separate tax due by completing “married filing separate” returns for each of you. (You will not file these returns).
3) Determine you and your ex-spouse’s share of the joint tax due (because neither spouse can receive a refund exceeding their share). Based on the results from steps 1 and 2, calculate your share using the formula below:
Divide your separate tax due by the total of your and your ex-spouse's separate taxes due. Then multiply that number by joint tax due to get your share.
4) Subtract your share of the joint tax due from your own withholding or estimated tax payments to determine your refund amount. File the amended return as “married filing joint” (because the filing status must match your original return).
Example
Mary and Bill filed a joint return with a Minnesota taxable income amount of $110,000. Of that amount, $75,000 belongs to Bill and $35,000 belongs to Mary. Their tax due was $11,000. The next year, they get a divorce.
Mary later realizes she failed to claim a $10,000 deduction on their joint return. She files a refund claim showing her taxable income for that year is now only $25,000 ($35,000 - $10,000). To calculate Mary’s refund:
1) Recompute their joint tax due based on $100,000 in taxable income (after taking Mary’s deduction). For this example, the recomputed tax due is $10,000.
2) Recompute the separate tax due by completing “married filing separate” returns for both Mary and Bill. For this example, Mary’s recomputed tax due is $2,250 and Bill’s is $8,250.
3) Determine Mary’s share of the recomputed joint tax due:

4) Determine Mary’s refund amount. The $1,000 change in tax due was because of Mary’s deduction, and the amount does not exceed her share of the joint tax due ($2,100). Mary can claim the entire $1,000 as a refund when she files an amended return, if she had at least that much in withholding and estimated tax. (She must file as “married filing joint.”)