Change the units the CAM Assist window shows lengths in. It doesn't change your part in your CAM package.
Switch units
Click the mm or inches button (Unit System) in the header at the top of the window, to the right of the stages. Each click switches to the other unit.
You can also pick mm or inches under Account > Preferences > Units.
Why did my units change?
Each time you start CAM Assist, it switches to the units of the part open in your CAM package. In Autodesk Fusion, that's the document's default length units. In SolidCAM, it's the CAM part's units. Your choice then applies until you next start CAM Assist.
If no part units are available, CAM Assist uses the last units you picked on this computer. The first time, that's inches if your computer's language is US English, otherwise mm.