You can try running TVH with the --noacl command line parameter. You can then get into the user/password details and rename/reset as required. Remove --noacl once complete.
https://docs.tvheadend.org/documentation/appendices/cli-commands
If it is a very new installation and you have done nothing with it, you could just delete your TVH configuration files from your home directory and re-run the Setup Wizard.
