These are instructions on how to manually change a password for an account in 365 via Windows Powershell. This is the only way to set a password specifically to something you want. Otherwise, when you reset a password through the Exchange Admin Center, it will randomly choose one for you.
Change 365 Password Instructions
The first thing you will need is the Windows Azure Active Directory Module for Windows PowerShell to connect to 365. It can be downloaded here:
The first thing you will need to do is enter in your 365 credentials.
$msolcred = get-credential
Now we need to connect.
connect-msolservice -credential $msolcred
Now you can choose the password and user you want to change, simply edit email@example.com and TheNewPassword.
Set-MsolUserPassword -UserPrincipalName firstname.lastname@example.org -NewPassword TheNewPassword
If the above information helped you, make sure to check out our official Office 365 Migration Guide.