Am I able to revert the changes I have made to my username?

The only way would be to change it again, which you would have to pay for.

i already have access to the account i just want to revert the username that they changed without paying for it

Hi there, I appreciate your response clarifying the situation.

I would like to confirm. Is only the Minecraft username the one the hacker changed it?

You can try to revert this by changing the username here:
https://www.minecraft.net/en-us/msaprofile

Otherwise, if that’s not an option for you, I strongly recommend reaching out to Minecraft support and review with them the options you may have regarding reverting this change or setting a new username.
https://help.minecraft.net/hc/en-us/requests/new

I also wish to correct some information. I am able to change some of the Xbox One/Windows 10 Online Safety settings. Some, but not all. I have been trying to change the first two options:
You can create an join clubs, and Broadcast Gameplay. Changes made to either of these options are reverted to Block, but every other option can be changed.

I meant username not gamertag

I couldn't find any information regarding username changing/reverting anywhere so that's why I'm worried they straight up don't allow it. I had to ask here.