From August 2010 you can renew your car registration from any other emirate, irrespective of where you bought the vehicle or where it was first registered. Before you can do so you will need to arrange your car insurance, which you can do here and now with InsureMe.ae.
For cars older than 3 years you will need a "passed car test certificate" which you will receive after a vehicle test at any of the authorized test centers (like Tasjeel/Eppco, Emarat Shamil or Belhasa's Wasal).
You can now renew your car registration online at the RTA's website if have an current car insurance policy certificate, a “passed” car test certificate and your old registration card. Please note that all your fines must be paid.
Don't forget to check and settle any outstanding traffic fines before you start registration renewal procedures! Even this can be done online on the webiste for Dubai Police.