Can I convert this wheel to fixed or free wheel?

Pretty useful info here

Conventional Freewheel-type Hubs
The cheapest way to convert a multi-speed bicycle into a singlespeed is to use the original rear hub, assuming that it is made for a conventional threaded freewheel. A single BMX freewheel will thread right on. Unfortunately, the chainline is likely to only work with your granny gear unless you re-space the rear axle and re-dish the wheel.

You'll need to remove the freewheel (cassette block) and replace it with a single freewheel.
Pretty much any local bike shop will be able to do this for you

*take it to a Local Bike Shop and not Evans etc // Small businesses are important