How do I change the amount I entered for federal income tax withheld

You should be able to go back to the source where the federal taxes were withheld that you entered incorrectly from to edit and/or review to make your changes.

Most sources of income should have a location to enter federal taxes if the ability exists and state taxes if the opportunity exists.  For example, most tax reporting documents that must be input generally have sections in them for federal and state withholding. This is referring to W-2s, 1099's, etc. as tax reporting documents.

For example, if you have self-employment income without a tax reporting document, then paying estimated federal or state tax payments are the only way to pay taxes in to cover that type of income for the year.

Your tax return is set up with an income section, a deduction and credit section and then finishing up sections.

You can always go into the tools section search bar and enter a topic for search so it will guide you.