Did you put a remote bomb or an ignition bomb on your vehicle...? That will prevent your vehicle from being upgraded... If so, you need to do the following...
First, confirm that your car has insurance... To do this, go into the garage in which that car is stored... Press down on the D-Pad to bring up the vehicle detail box (which will appear over the vehicle like a player name)... Make sure that it says "Insured" in that box... If it says "Not Insured", you need to go buy insurance...
Once you are sure that the car has insurance, use the bomb you purchased to blow up your car... If it was an ignition bomb, you will need another player to try and drive off in your car while vehicle access is set to "Everyone"...
After the bomb has been used and the car is destroyed, call Mors Mutual Insurance, and get your car back... Now the bomb should be removed, and you should be able to access upgrades for your vehicle...