Go to Destinations, open the menu on the destination you want to remove, and choose Remove. Confirm and it's gone.
What happens to aliases pointing to it
Any alias that was forwarding to the removed destination loses its forwarding target. Those aliases don't disappear — they still exist on your account — but they stop delivering email until you assign them to a different destination.
To fix affected aliases, go to Aliases, find each one, edit it, and select a working destination.
You can't remove the default
If the destination you want to remove is currently your default, you need to set a different destination as the default first. See Setting a default destination.
You can't remove your only destination
If it's the only destination on your account, you'll need to add and verify a new one before removing the existing one. You always need at least one destination so there's somewhere for your aliases to forward to.
Before removing
Check which aliases are assigned to the destination you're about to remove. The destination row shows an alias count (or you can filter the alias list by destination). Reassign or delete those aliases before removing the destination to avoid any forwarding gaps.