I filed both Federal and State as "Married Jointly". I want to amend both to "Married Separately". I have to file Form 8958 for state. How do I enter my spouse's income?

Be aware, that changing from MFJ to MFS can only be done at the federal level by the due date of the original return. After that, the change is not allowed.

About Form 8958, Allocation of Tax Amounts includes the full instructions for the form but you need Publication 555 Community Property - Internal Revenue Service will help you navigate issues. It has examples and helpful information. It states:

Generally, community income is income from:

• Community property;
• Salaries, wages, and other pay received for the services performed by you, your spouse (or your registered domestic partner), or both during your marriage (or registered domestic partnership) while domiciled in a community property state; and
• Real estate that is treated as community property under the laws of the state where the property is located.