Welcome to Retool @Jessica_D!
You do (for now) have to write some JavaScript to update the records in your MongoDB, but it shouldn't be too much more complicated that what you've written so far.
I'm admittedly not super familiar with the MongoDB resource connector, but I believe the quickest implementation would be to use the bulkWrite action type, and write some JavaScript to transform the changesetArray into a set of updates for MongoDB to perform. Something on these lines maybe:
Note that the records in changesetArray will include the primary key of the row, along with any changed values. You can change this if you need to reference other values on the record in the Advanced Interaction section:
To fully set-up a bulk write-back flow, you'll want to connect the "Save" action of the Table to trigger the query:
To configure a placeholder look for the "Placeholder" Add-on in the column inspector:
Please let us know if you run into any other points of confusion as you finish building your first tool!