Nice command can be used to start a process with low/high priority:

nice -n command

value = -20 for highest priority and 20 for lowest priority

To reset the priority of a running process, find the pid of the process and run:

renice renice +1 102 -u daemon root -p 132

would change the priority of process ID’s 102 and 132, and all processes owned by users daemon and root.