Go back on the same path.
Alternative way back (45 minutes longer)
Go back down by Lac Merlet Inférieur until you reach the crossroads under the Merlet Lake shelter. Take the path opposite down to the Grand Plan shelter (15 minutes). To reach the Col de la Platta from the Grand Plan shelter, retrace your steps for 200 metres then go up the path along the edge on the right back to the original path.