Umm, first take your wheel off. No idea if you know how to do that, but there is like two nuts on eahc side of your wheel holding it in. get a spanner and hold one side of it, and get a rachet and unscrew it then pull it out, There are two bushes on each side of the wheel. REMEMBER in which order they go.
Next once you have done that, I have like a pry[wrong spelling] bar which I stick under the edges of the rim to pop out the outside plastic?[the bit with the knobbies] keep doing this around in a circle until it has all popped out. Grab ur tube out and fill it up with air, stick the tube in a bucket of water and check where their are any holes. You may choose to get a new tube or get a tube repair kit [pretty self explainatory].
Then pop it all back in [should be easier this time].
I hope this helped,
Its only how i do it and i dont know if i worded it properly!
Benox