Install indicator-cpufreq by entering the following commands in the terminal:
sudo apt update sudo apt install indicator-cpufreq
Description:
CPU frequency scaling indicator
Indicator applet for displaying and changing CPU frequency on the fly. It provides the same functionality as the GNOME CPU frequency applet, but doesn't require GNOME panel and works under Unity.
Homepage: https://launchpad.net/indicator-cpufreq
Version: 0.2.2-0ubuntu2
Section: universe/python