The Apply to past data button, in the top-right corner of most settings sections, lets you choose two things: how many days to overwrite (up to 45) and which platforms to reprocess.
Why choosing the platform helps
It's useful, for example, after reconnecting an ad account: you can reprocess only Meta without touching the rest of your settings or your store data again.
Remember that the changes you save apply from that moment forward. To make them affect orders that are already synced, you have to use this button.