At the moment there is no easy way to do that since there is no way via the Kops API to create a secret of type "Secret" (quite confusing I know). The only workaround is to change the credentials, in this case your password directly on your s3 configuration as explained here:

https://github.com/kubernetes/kops/blob/master/docs/secrets.md#workaround-for-changing-secrets-with-type-secret

and force a rolling-update of your cluster by changing its configuration (I just updated to the new version of k8s for example) or simply run kops rolling-update cluster --yes --force

Apparently this will be addressed soon by one of the future releases.