To set a new value for mileage counter, use the command unit/update_mileage_counter:

svc=unit/update_mileage_counter&params={"itemId":<long>,
					"newValue":<uint>}

You can find an example of this request in the sample Creating, editing and deleting items.

{
	"cnm":<uint>	/* value of mileage counter */
}