Shouldn't you rather investigate why the Memory Usage is this high in the first place

Anyway, to answer your question:
If you want to change the threshold for all Devices in a specific Device Class, then you should do this by creating a new Monitoring Template for said Device Class (you can copy an existing Template by choosing "Copy / Overwrite Template").
However if you want to change it for a specific Device (I wouldn't recommend that), then you could also create a local Template (select the Device and choose "Add Local Template").

Hope that helps.