To modify the execution priority of an already running process, you cannot use nice but should use renice. The basic usage of renice is as follows: renice # PID ("#" is the priority, ranging from -20 to +19, where smaller values indicate higher priority). Only the superuser can set negative priority values.
Example:
| $ renice 19 1234 ← Change the priority of the process with PID 1234 to 19 $ renice +1 5678 ←Increase the priority of the process with PID 5678 by 1 |