How to change the password on linux 18.3 on a dual boot linux 18.3 W10

i have a problem with linux 18.3. I wanted to change the the password which seemed easy enough as you had to choose preference and alter your code. Unfortunately, if you can modify the name you can’t modify the modify “switch” as it remains grey and useless.
I tried the terminal but there again it was a failure as unix did not accept the new codeand mentioned an “Authentication error”.
I am on a dual boot linux 18.3 and w10. Can anyone help?

Linux Mint 18.3?

Actually, GNU/Linux.

That means you provided the wrong password to be able to change your password.

What exactly have you done in the terminal?