We are using AD connect to sync users/passwords from on premise but we need to change the username and or password of the account used on the AAD connector.
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Haven't tested but this should work:
1. Create a new account for sync and give it a "Directory Synchronization Accounts" -role
New-MsolUser -UserPrincipalName firstname.lastname@example.org -DisplayName "Sync Account" -Password yourpassword -PasswordNeverExpires $true -ForceChangePassword $falseAdd-MsolRoleMember -RoleName "Directory Synchronization Accounts" -RoleMemberEmailAddress email@example.com
2. On AAD Connect computer, run the following command and give the new credentials
See here for more details