Get the GEE script for downloading DEM below
var srtm=ee.Image("USGS/SRTMGL1_003").clip(ROI);
Map.addLayer(ROI);
//Map add layer
//var srtm_clip= DEM.clip(aoi);
Map.addLayer(srtm, {min:50,max:3000}, 'DEM');
//Export Image to drive
Export.image.toDrive({
image:srtm,
description:'Free_state',
region:ROI,
folder:'earthengine',
scale:30,
maxPixels: 1e13});