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If you click on the eye icon on the top bar and select 'all'

Then go into Contexts on the left menu bar and all contexts are are available on the right, from here you can edit by clicking on the context.

I'm not sure what would happen if you deleted a context which still had items in it, I assume they would have no context, but someone else may be able to help.

Rember to click on the eye when you've finished and set it to whichever option you want them to.