Replace any string matching the pattern of a MAC address (([0-9A-Fa-f]{2}:){5}[0-9A-Fa-f]{2}) in the file /etc/upt.conf with the string j3q45D5/g.
sed -r 's/(([0-9A-Fa-f]{2}:){5}[0-9A-Fa-f]{2})/j3q45D5\/g/' /etc/upt.conf