Following steps could be referred for the same:-
a.. First stop MySQL, Apache, and cPanel to prevent writing to the /tmp partition
b. Take a backup of the current /tmp folder.
c. Umount /tmp partition. If you’re unable to, you can do an lsof to see what processes are still writing to it, and kill them off.
d. Delete /usr/tmpDSK
e. Now edit the file /scripts/securetmp. Search for “tmpdsksize” and set the size you want in MB.
f. Save and quit the file.
g. Now you can untar the /tmp backup
h. Start all the services.