Welcome to FT and the EK Forum. The amount of miles you need to upgrade from J to F between HKG and DXB will depend on whether or not you hold a flex or saver ticket. If it's a flex fare, 32,500 miles and if a saver, 42,500.

All the information you need can be found on Skywards.com

Best to call your local EK office to see what type of ticket you hold, but if you can see your booking online, you should be able to tell quite easily.

I've checked the flight and it doesn't look good (EK381), given that Business looks sold out and there is only 1 F seat currently for sale in A/P class. Don't be too disheartened though as the load could fluctuate a lot between now and then...