For video will be under the video menu. For stills under the photo menu. I have it set to a shortcut in the quick menu for easy access. Although under certain conditions you may find it greyed out/unavailable for selection, with the gh3/4 at least. Not sure about gh5.
Best to check the manual for the specific model in question for the specifics, all this information will be in there. The pdfs can be easily found via google
Example, for GH4:
Video done by panasonic on setting up ETC mode for GH5.
In this Video Panasonic's Tech Team Provides you with a quick explanation on how to set and use the Teleconverter function. This function allows you to raise the telescopic effect. Extended Tele Conversion enables you to take pictures that are further enlarged without deteriorating the picture quality.
The model shown in the video is the LUMIX DC-GH5, the information in this video also applies to LUMIX DC-G9