How can I update Turbo Tax from Head of Household to Single to get the credit for other dependent?

You might be confused about the best filing status to use.     If  you are filing as HOH, you get a higher standard deduction ---which makes more of your income "tax free"---why would you want a standard deduction of $14,600  filing Single instead of  $21,900 filing HOH?   That would not make sense.     You can file as Head of Household if you have a qualifying dependent.   Does your son qualify as a qualifying relative dependent?  He is too old to be a qualifying child but if he had less than $5050 of income not counting Social Security ---and you paid for over half his support, he could be a qualifying relative for the $500 credit and you could use him as the qualified dependent to file HOH.

Qualifying relative

• They don't have to be related to you (despite the name).
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They lived with you the entire year (exceptions apply).
• They made less than $5050 in 2024 (not counting Social Security)
• You provided more than half of their financial support.