If you want to update a different record in an app every time your Zap runs, you'll need to use a search step to find that record first. Search steps are a way to find existing data in your apps that later actions in your Zap can reference and use.
For example, you might want to set up a Zap so that every time you receive a new Google Forms submission, it adds a comment to a different Trello card depending on how the form was filled out. In order to update an existing Trello card, a search step must be used to first find that card.