Update an existing cluster with latest API version :
If you have the previous template available you can redeploy only the Service Fabric resource with required API version or get template from the deployment section of your resource group otherwise simply you can pull the latest template with https://resources.azure.com
Navigate to your subscription by expandingsubscriptions-><Your Subscription>->resourceGroups-><Your Resource Group>->providers->Microsoft.ServiceFabric->clusters-><Your Cluster Name>
Copy the template
Create a new custom deployment template and deploy via Azure portal or powershell/ VS with making the changes for the API version.
Important note for the template copied via Azure Resource Explorer make sure your remove the etag and cross verify your existing Fabric setting applied or update any setting want to apply which requires latest API version and not able to perform via Azure Resource Explorer.