You can edit or delete a Focus Session whenever it isn't running. A running Soft session needs ending for today first. A running Hard session can't be changed until it ends.
Edit a session
On Home, tap the Focus Sessions card, then tap the session.
Tap Edit Session.
Change the name, apps, times, days or difficulty.
Tap Update.
Delete a session
Open the session and tap Edit Session.
Tap Delete, then tap Delete again on the "Delete this session?" screen.
Deleting can't be undone.
If the session is running
Soft: tap End Session to end it for today. Then tap the pencil next to Resume Session to edit or delete it.
Hard: wait until it ends. Once it's outside its hours, Edit Session works as normal.
Changing a folder later doesn't change a session you've already made. To update the apps in a session, edit the session itself.