How do you apply a state refund to next years taxes?

It depends upon the state.  Each state has a slightly different set of questions as you go through the state interview section of the program.

I recommend going through the state interview screens again.  If you have a state refund, you will typically be asked whether you want to apply the refund to 2025 in Other situations.

For example, in these screenshots for California, you would be given the option to make changes by selecting edit to the right of "extensions, estimated payments and contributions."  That would then take you to the next screen where you are able to select "apply refund to next year" which is where you would make your entry to apply all or a specific amount to 2025.