From MorphOS Library
ChangeTaskPri - Changes the priority of the given process
PRIORITY - new priority of the process PROCESS - number of the process to change priority
The range of acceptable values for PRIORITY is the integers from -128 to 127, with higher values yielding a higher priority (a greater proportion of CPU time is allocated). However, do not enter values above +10 to avoid disrupting important system tasks.
NOTE: use the Status command to display the current process numbers.