Massachusetts state taxHow do I change line 38b (income tax withheld from 1099) ?

What kind of 1099?  Everything is entered on the federal side then flows to the state.  Go back to your 1099 entry.

Edit a 1099R under
Federal Taxes
Wages & Income
Then scroll way down to Retirement Plans and Social Security
Then IRA, 401(k), Pension Plan Withdrawals (1099-R) – Click Update or Revisit

State withholding is in 1099R box 14

If you took a picture to enter your document.....DON'T.

Either enter the data manually,

OR

...or make absolutely sure you "Edit" every document you do enter by taking a picture, because all kinds of entry errors are introduced by picture-taking.  That AI procedure is not really ready for flawless use.

Same if you imported it.  Go back and delete it and enter it again MANUALLY.