That’s quite easy:

• go into the installation folder
• find pg_hba.conf file and open it
• modify the authentication method to trust. (for me it was set to scram-sha-256)
• restart the PostgreSQL server (in Windows: Run > Service > Find PostgreSQL, right click and select Restart)
• try logging in without user & pass from SQL Shell (in Windows)
• set/change password: alter user postgres with password ‘your_new_password’;
• exit
• go into the pg_hba.conf file and change back the authentication method