How to delete/reset database?

I guessed that "postgres" is the database that shoudl be used.
before restorign I have
dropdb -U postgres --if-exists postgres -w -h "$SUPA_URL"

to which I get

dropdb: error: database removal failed: ERROR: database "postgres" is being accessed by other users
DETAIL: There are 4 other sessions using the database.

There's no 4 users! I am only one and I am not accesing it.
I couldn't find any option to reset or even erase it. Do I have to delete whole project?
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