I want to amend my state taxes because I forgot to claim an EV tax credit, which I see is box number 24 on the form. Should I just fill that part out since that's the only change to my taxes? Or should I fill out all of the other boxes as well even though nothing else has changed?
Fill out the tax return entirely with the correct values through line 29 (for 2020, line 31 for 2021), at the end (line 31 or 33) you can optionally enter the amount overpaid or amount owed from your initial return (or latest amendment, if amending again). Then subsequent lines are used to calculate whether the net change from your previous return. If you leave it blank as the note indicates they will compute it for you.
An important note that trips people up sometimes:
You must submit all required documentation with this return - even if you submitted it with your original return. This includes all supporting forms and schedules as well as copies of certificates. All attachments and certifications must be included with the amended return even if there is no change to that credit or tax attribute
The above is for the 2020 104X, due to the sales tax refund for 2021 the section pictured starts on line 34. Ensure you're filing the 104X for the proper tax year.