What I did to resolve the same problem was:
Open pg_hba.conf file with gedit editor from the terminal:
sudo gedit /etc/postgresql/9.5/main/pg_hba.conf
It will ask for password. Enter your admin login password. This will open gedit with the file. Paste the following line:
host all all 127.0.0.1/32 trust
just below -
# Database administrative login by Unix domain socket
Save and close it. Close the terminal and open it again and run this command:
psql -U postgres
You will now enter the psql console. Now change the password by entering this:
ALTER USER [your prefered user name] with password '[desired password]';
If it says user does not exist then instead of ALTER use CREATE.
Lastly, remove that certain line you pasted in pg_hba and save it.