If you want to modify an RDS PostgreSQL server, if you want to turn on slow query logging for instance, you will need to modify the parameter group of that server. Normally, you just edit your parameter group and AWS will handle the rest with no reboots necessary.
If, however, you’ve decided to use the default parameter group, you will find that you cannot edit the file. You will need to replace it with a custom file. This does require a reboot. Thankfully RDS does a rolling reboot when in multi-AZ mode, but it was more of a task than it would have been if we had started with a clone of the default group.
TLDR; Always use a custom parameter group for RDS, even if it is just a clone of the default one.