Changing reminder frequency in mass for existing expirations can be done using the bulk edit panel.
To change the reminder frequency for multiple expirations, first select the expirations by clicking on Expirations in the top menu and then selecting the ones to be changed using the checkboxes on the left or click the top left checkbox to select all of them.
You can also do a search or filter to look for the expirations that need to be changed.
After doing this, the bulk edit panel will show up. In the panel, under Reminder Frequency, select Replace existing value with… and the enter reminder frequency required.
After this, click on Update.