If the project is in MM and you want to show the dimensions in M in certain views, you can create a new dimension type that has its units set as M.
However, to change the Level dimensions in Elevation views to show in Meters, you have to edit the Level head family (look for it in the Project Browser > Families > Annotation Symbols > Level Head…).
In the family editor, select the ‘Elevation’ label and select in the ‘Edit Label’ button in the option bar.
Select the Elevation parameter in the RIGHT side of the Edit Label dialog box. (If you select the label in the LEFT side, the “Edit parameters Unit format” button - small hand like thing at the bottom – won’t become available.
In the format dialog box, you can uncheck the ‘Use project settings’ and change the units.