Open You → Plans & billing and find the group under Your slots. Choose Stop covering to schedule its coverage to end.
The group keeps its paid features until the current billing period ends. Its slot stays occupied until that date, so you cannot immediately move that slot to a different group. On an annual subscription, this can mean waiting until the annual period ends.
If you change your mind before coverage ends, choose Keep covering beside the same group. That cancels the scheduled stop. Any genuinely empty slots can still be filled straight away.
Stopping coverage for one group does not cancel Together or remove you from that group. When coverage ends, that group returns to Free unless it has other valid coverage. Read what happens to its photos and features.