Because the Edge Agent is a docker image, upgrading to the latest agent is just a matter of executing the docker pull command, and then restarting the agent. The latest tag on our Edge Agent always refers to the latest version we have released, but we also have explicit tags for each individual version. You can read a little more about it here.
As far as the target edge agent version for a workflow, you can read more about it here. You are allowed to “upgrade” the target edge version on a workflow, but never “downgrade” it (as we can’t easily tell if you are (or are not) using features in the workflow that would only be available on newer versions). When you create a new edge workflow, you can pick the initial target version, and it can be as old as you want.