Try using below.

(function transformRow(source, target, map, log, isUpdate) {

gs.log('Source priority is ',source.priority);//check in logs
 
if (source.priority== 'High') {
target.priority ='1'; //make sure  you pass the dictionary value here for priority
}else if (source.priority== 'Medium') {
target.priority= '2';// make sure  you pass the dictionary value here for priority
}else if (source.priority== 'Low') {
target.priority='3';//make sure  you pass the dictionary value here for priority
}

// Add your code here

})(source, target, map, log, action === "update");