You can trek, drive or fly to Upper Mustang. The starting point for all is Pokhara. If you want to fly, you need to fly to Jomsom. Similarly, it takes 8-10 hours by car/bus from Pokhara to Jomsom. Then you can fly from Jomsom to Lo-Manthang by helicopter.
Alternatively, you can hike from Jomsom to Upper Mustang or ride a jeep. Depending on the route, trekking takes 4-5 days, but jeep driving takes a day. If you want to explore Annapurna Conservation and Upper Mustang all at once, you can start trekking from Tikhedhunga, the starting point for Annapurna trekking.