Several situations allow you to change insurer:
When your contract expires: you can terminate your insurance either with 2 months' notice or upon receipt of your expiry notice under the Châtel law.
At any time during the year if you are eligible for the Hamon law (or Consumption law). You are eligible for this law if you have been insured for one year or more with an insurer.
At any time when you change your car following a sale or a destruction, for example. You just need to provide proof of your situation to your insurer to terminate the contract.