windows 10 - Office 365 and office 2013 side-by-side

I have a recent dell XPS 13 (i7) running windows 10 and 4K internal and external monitors.
I have noticed that some programs run very slowly when they have to use 4K screen space. Office 365 is one of them. This is a known issue which up to date a solution has not been found. I'm unsure if this solvable by running higher end hardware (eg external graphics cards)
Anyway, I have office 365 running alongside office 2013 for the simple reason that outlook 365 has some enhancements which I really appreciate (it remembers the zoom level for reading messages) and also does not go DPI crazy when I move from my external screen (which is hoked up to a Dell docking station).
Anyway, the only inconvenience I am currently finding is that no matter what I do, the default programme for opening office files (word, powerpoint, excel etc...) is office 365. Even when I go through the process of Right Click>>Open With>>Choose Another App>> and then nagivate to the office 2013 folder (office 15) and pick the 2013 version of the programme, the file will open with the respective office 365 programme! This is incredible!
Anybody know how to hack the default file associations?
Regards,
KL