Editing Value of DB From a Different DB

Looking for some advice creating an "application" in Notion. I have a page open from a database (DB 1), and another smaller database (DB 2) within this page. I'd like to take the sum of a column from DB 2 and make it a value in DB 1. I can't figure out how to make this possible. Attached a screenshare to try and explain my hope.

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