If you recall the old password please follow step A,
if you don't recall the old password follow step B.
A.
>Click on the Apple logo from the top left of your screen
>In the drop-down select "System settings..."
>Once in settings, locate "Users & Group"
>Cilck on your account profile or "i" icon, and select "Reset Password"
>Enter the old password then the new password
>Once the password has been changed, restart your device and try to install the software
>Make sure you're connected to the Internet
B.
>Restart your device
>While it's booting up, hold the "Command" key and keep pressing the "R" key until the Mac Logo boots up on the screen (this will boot MAC into the recovery mode)
>Once in recovery mode, from the top left locate "Utilities", click and select "Terminal"
>In the Terminal window enter resetpassword all in one word, and hit the "Enter" key
>Select your account and enter your new password
>Then close the terminal, click on the Mac logo from the top left corner, and restart your device