Hi @William_Sweet,
You can use the Batch Update extension by Airtable to quickly update records contained within a view. I would create a temporary view, filter it based on something existing in your data to select the records you want to update, then update records in this view using the "Add/remove multiple choices" action of the Batch Update extension.
Another less fun by possibly bearable option is to utilize the keyboard only by beginning to type the multi-select value, then hitting Enter once it appears, and moving with the arrow keys.
Hope that helps!
-Stephen
-Stephen