I was curious about that too, so I looked on Forge. There is a behavior called "iPad Resolution OpenFL" by Max Glocking, which shows how to get the current resolution in code: openfl.system.Capabilities.screenResolutionX or Y.
You should find the code on how to SET the resolution in the openfl API, and I guess you're done. Hopefully.
I am not a coder at all, so if you or somebody else finds out how to set resolution at runtime, I would like to know. Makes desktop games very professional.