Octopus generally regards a release as read only. There are no options to alter or define variables during a deployment. A prompted variable is the exception as it allows you to set a variable value after release creation but just before you deploy.
So unfortunately, you won’t be able to achieve what you are after here with editing variable values during your deployment. The idea is that each release is regarded as a source of truth for what is being deployed and what exists on your targets.
As deployments in Octopus are designed to be an automated process, perhaps I can help you resolve the underlying issue you are trying to resolve, removing the need to modify your deployment after it has started.
If you could provide some information about your goal and requirement for changing the deployment midway through, I may be able to help you configure Octopus to better meet your needs.
Let me know what you think.