The option you are looking for is drawing_units. It is located in the Miscellaneous options group.
Be aware that after changing the option from inch to mm you will need to also update the values for all options that are referring to linear sizes such are text_height or draw_arrow_length.
You can also load Iso.dtl located under <Creo load point>\CommonFiles\Text directory and start from there.
If this is something you need to do often, I suggest to save the drawing options to your library and to set the config.pro options pro_dtl_setup_diror drawing_setup_file.
You can also apply the DTL file to your drawing template. This way, it is possible to have different templates with the different DTL options and you can choose the required one each time a drawing is made.
