Both destinations are served by Dehra Dun (DED). Since there's no flight route between them, the best way to travel is by road.
Valley of Flowers
Fly to Dehra Dun (DED), then travel by road to Valley of Flowers. Taxis, buses, and private cabs are available from the airport.
Dehradun
Fly to Dehra Dun (DED), then travel by road to Dehradun. Local transport options include taxis, auto-rickshaws, and state buses.