Niceness values range from -20 (most favorable to the process) to 19 (least favorable to the process). To put this simply, the negative values (Eg. -20) gives higher priority to a process and positive values (Eg. 19) gives lower priority.
Kill the process with with kill command:
kill -9 13957
Again, start the process with niceness value of 15:
You can also use -n option to increase or decrease the priority.
To increase priority, run the following command as root user:
# nice -n -15 cmdname &
To decrease priority, run the following command as normal user:
$ nice -n 15 cmdname &