If you're wishing to disable your Android phone or connected app on your Google Fit account as an activity tracker you need to "un-allow" the Active for Good app to read your "Step" and "Active Energy" data from Google Fit.
How to disable (this is assuming you have the most updated version of the app):
- Locate & open the "Fit" app on your Android device
- From the main activity screen, select the navigation menu (top left)
- Select "Settings" from the navigation list
- Select "Connected apps" from the list of settings
- Locate and select "Active For Good" on the list of connected apps
- Tap "disconnect" to un-allow the app to receive your data
The next step is to add another tracker. See here how to do that.