You have to go to: Settings > Calendars >
{calendar} …
menu > Manage reminders
So many steps, making such a useful feature very hidden.
I just found that this exists because @Danny said so in a comment in https://morgen.canny.io/feature-requests/p/set-default-reminders
When editing individual task/event reminders, if they're equal to the defaults, the disabled
Restore defaults
button could be replaced by an
Edit defaults
button. This would let users know that there are defaults and that they can be changed, without compromising on screen real estate.