[LINUX]
might work for windows too
After installing postgres follow following steps in order to setup password for default system account of Linux execute following in terminal:
user:~$ sudo -i -u postgres
postgres@user:~$ psql
after executing above two commands you will get into postgres shell
Execute this query in postgres shell:
postgres=# ALTER USER postgres PASSWORD 'mynewpassword';
your new password is 'mynewpassword' without quotes and now you can connect with external GUI tools like DBeaver