Mass Update Records from a Related List
Melody from Salesforce Flowsome has done it again with a fantastic pair of posts that cover a few different ways to select and mass update records from a list view. These well written articles are easy to follow and present step-by-step how you can implement this functionality using Flow.

Use Case: Mass Update Records From Related List #1 (ids Variable) (salesforce-flowsome.com)
Use Case: Mass Update Records From Related List #2 (ids Variable) (salesforce-flowsome.com)