Changing default user for PostgreSQL on Windows 10

createdb is a command that has to connect to the server just like any other client command (even when run on the server itself), and as such requires a database user to connect with.

If you don't specify a database user with -U username option, it takes your OS username, hence the reference to "R-61" in the error message. The password that is asked is the password of a non-existing R-61 database user.

There are also other options such as the host defaulting to localhost and the port defaulting to 5432, see its manual page for all of them.

Since you mention that you know the postgres superuser password, the simplest solution would be to use the postgres user to initiate the command. It makes sense anyway, because on a fresh install this user would the only one that has the permission to create a database.

Try createdb -U postgres newdatabase

It should ask for a password, and that will be the password of the postgres database user.

If it doesn't ask for a password, it means that the security policy in pg_hba.conf is configured that way. If the password is lost (or no password has ever been set, which is the case for some automated installs, especially on Unix), pg_hba.conf has to be changed and the service reloaded to authorize passwordless connections, at least temporarily to set a new password.

You change the directory (cd) to the one where you downloaded the PostgreSQL installer to and just type in the name of the executable and hit enter.

As an example:

cd C:\Users\<User>\Download\
PostgreSQL-Installer.exe

Or just use the whole path (be careful with spaces):

C:\Users\<User>\Download\PostgreSQL-Installer.exe

Another option would've been to rightclick while holding Shift and selecting "Run as different User" or "Run as administrator". Both methods might not working depending on whenver administrative priviliges or a separate user account are actually needed.