Press r while running top and enter PID to renice. After that - enter the nice value.
You can change the process priority (PR field) by modifying the nice (NI field) in top.
The nice value is the value that it is added or substracted to the priority. It goes from -20 to +19.
Lower value means that the process is becoming very aggressive - it claims a higher priority.
Higher values means that the process will leave resources to the other processes (less aggressive).
If you are normal user, you can't increase the priority, you can only decrease it.