If you suspect you are currently running a nulled version, the safest course of action is to perform a fresh installation of a legitimate OS and WHM/cPanel on a clean server and migrate your backups. The Hidden Dangers of Using a Fraudulent cPanel License
A "nulled" script is a piece of software that has had its licensing and verification features bypassed or removed. In the case of cPanel/WHM, which typically requires a monthly subscription, these scripts claim to give you full "Admin" or "Root" access to the hosting panel without paying the official licensing fees. The Risks of Using Nulled Control Panels cpanel and whm full nulled by scriptmasters
Experts and developers often warn against these "ScriptMaster" releases for several reasons: My License horror story - cPanel Support If you suspect you are currently running a