A managed instance group needs a cautious rollout for a new, lightly tested instance template to minimize user impact. How should you update instances?

The correct approach is to perform a canary update. This method allows you to deploy the new instance template to a small subset of instances first (the "canary" instances). By setting a target size for these canary instances, you can monitor their performance and verify the new feature without impacting the majority of users. If the canary deployment is successful, you can then roll forward the update to the remaining instances in the managed instance group. This minimizes risk and user impact. Manually patching instances is not scalable or automated for a managed instance group. Performing a full rolling update immediately is risky for a "lightly tested" template, as issues would affect all users. Deploying a new instance group for canaries is an option, but a canary update within the existing managed instance group is a more direct and efficient way to achieve the same goal.