There are several situations which allow you to change your insurer:
At the end of your contract: you can terminate your insurance either with 2 months notice or upon receipt of your notice of termination under the Châtel law.
At any time during the year if you are eligible for the Hamon law (or Consumer law). You are eligible for this law if you have been insured for one year or more with an insurer.
At any time during a change of vehicle following a sale or destruction, for example. All you have to do is provide proof of your situation to your insurer to terminate the contract.