2) Navigate to the frequency and gain menu and make sure it is highlighted.
Frequency and Gain
Or, like this when IXR is enabled.
Frequency and Gain
3) Press the Go button and page 1 of the frequency and gain menu appears.
Frequency, Gain & Inhibit
4) Press the right-navigation button to highlight the gain menu.
Low/High Gain
Thermo Scientific APEX User's Guide
Page 53