Once you've been accepted at your Firm choice university, the Insurance choice is no longer valid, even if it shows on UCAS Track that you're Unconditional there – status UI. This is the case even if you didn't get the grades required for your Firm choice, but were accepted anyway. The Firm choice university can only release you into Clearing, it can't release you to your Insurance choice.
If you'd prefer to go to your Insurance choice, you should contact the Insurance university to see if you can be accepted. If the Insurance university agrees to accept you, you should then contact your Firm choice and ask to be released into Clearing, or release yourself by clicking the 'Decline my place' button on your UCAS Track page. Once you've been released into Clearing, you'll be able to formally re-apply to the Insurance choice via Clearing.
Remember that the Insurance choice is under no obligation to take a student who's Firm at another university, so you should make sure you'll be accepted before getting released from your Firm choice. You may also wish to check with your Insurance choice whether any guarantees about accommodation would still apply to you as a Clearing applicant.