What command changes the priority of a running process?

The command that changes the priority of a running process is renice.

renice adjusts a process’s niceness value while the process is already running. On Linux, the usual target is a process ID, as in renice 10 -p 1234, although the command can also operate on process groups or users. A higher niceness value makes a process more willing to yield CPU time; a lower value gives it comparatively higher scheduling priority.

The main trap is confusing renice with nice. nice starts a new command with a selected niceness, while renice modifies an existing process. The command chrt is different again: it changes real-time scheduling policy or real-time priority rather than ordinary niceness. Ordinary users generally may adjust processes they own, but increasing priority by assigning a negative niceness usually requires elevated privileges. The change affects the current process state and is not automatically a permanent service configuration.