I wanna change a child's account to an adult's account.

From my knowledge, an adult account cannot make a child account have a profile picture. Meaning the account you wanna change the profile pic of will have to wait till the account is deemed an adult account (What that means is is that the birthday on the
account reaches 18 years old).

I can't remember 100% but I am sure whoever set up the child account can change it. Like if you made an adult account on the xbox before making the child account that adult account can change wether the child account is a child account or and adult one.
I will just test this out myself now and post how it went

You can change a child account to an adult account, but you can't change an adult account to a child account. In order to manage them, you would have to create new child accounts for them.

Is the account in question a child's or adult's account? If its a child account you may want to change it to an adult account and give it a shot.

First question, are you using a child or an adult account.

I ask because if it's a child account, you can't change them without an adult account