It will depend on whether the life policy was setup to pay out on the first death or the second death.
If it is setup to pay out on the second death, then you will need a new policy (one written to pay out on the first death).
If it is setup to pay out on the first death, then you should be able to change the nomination of who should receive the pay out by letter. Just write to Aviva and say that you are retracting any previous nominations made and now wish that, in the event of the death of either one of you, you would like the payment to be made to the survivor, or if the survivor dies before the payment has been made, to your mother (or anyone else you now want to include).
Aviva should keep this letter on file and follow your wishes in the event of an untimely demise.