You don't say what you're running tvheadend on, but, assuming it's a regular distro, have a look at the files in/home/hts/.hts/tvheadend/accesscontrol - usesudo su hts if you need to (or equivalent). You'll find one file per access control rule - change or delete these from the command line to change access to tvheadend."admin": 0,
Many of the fields in the files are boolean, so you can add admin rights to that user by changing
"admin": 0, to
"admin": 1,.