It might be possible to change this by some method I am not familiar with, but I think the easiest and most straight-forward process is uninstall and reinstall.
Uninstall / Reinstall should not be needed. With FM server application completely stopped, go to Services and stop the FileMaker Server service. Then in the properties of the service, change it to run under your domain account.
You will need to ensure that the domain account has the proper privileges on the FM server machine. I can't remember if the domain account needs to have Admin rights on the machine or not. The last time I did this, the domain account was an Admin, but I can't remember if it was like this due to necessity or the Windows Server team just granted this priv as a standard thing.