So there are two ways to correct this, easiest is to use the info button at the bottom right menu bar, clicking on this open up the model info menu, at the top are the imperial units, and for some reason the default, under this the metric and for me mmm use, clicking on this even when half way through a model will switch it over.
Of course, just adding the dim in the measurements bar with mm after the number will work, but best to change the units completely.
Alternatively and possibly good practice is to open the three bar menu and make a new model and choose the metric mm template. whatever way checking the measurements bottom right is always good practice.
The free version has a download button and is the way my students can pull the saved version from Sketchup's cloud to the desktop and then into Moodle, my universities all singing communications platform. I make it quite clear to all, saving as metric mm is one of the marking criteria, and worth 10% of the marks, mean I know but it gets the point across.