NOTE 2020
see Affinity Help:
https://affinity.help/publisher/en-US.lproj/index.html?page=pages/Pages/masterPages.html?title=Master%20pages
"To detach or link a master page"
In my opinion the text in the help-file is not 100% clear - you have to right-Click on a top-level-layer to get the detach option.
- the missing point: now you get an orange bar at the top of the window,
As long as this bar is active, you have to do the edits, you click finish as soon as you want to leave this special "mode".
Just to make this topic more visible:
In Indesign the specific workflow is described as "Override master page items"
https://helpx.adobe.com/indesign/how-to/override-master-pages.html
normally done with a shortcut left klick with Command+Shift keys (macOS) or Ctrl+Shift keys (Windows) - this won t work in Affinity.
hope that helps - by the way it would be very cool:
-> topics and answers would be marked as "solved"
-> solutions would be transfered to the help
-> come comment-Functionality in the help, to improve it
best - Tom