The following script can be used to set new password in Office 365. Replace username and password of service account and save file as Set-Password.ps1.

Set-Password.ps1 -UserPrincipalName [UserPrincipalName] -NewPassword [NewPassord]

param
(
   [parameter(Mandatory = $true)][string]$UserPrincipalName,
   [parameter(Mandatory = $true)][string]$NewPassword
)

function Set-Password()
{
   # Connect to service
   $Username = "admin@mydomain.onmicrosoft.com"
   $Password = ConvertTo-SecureString "P@ssword" -AsPlainText -Force
   $Credentials = New-Object System.Management.Automation.PSCredential $Username,$Password
   Connect-MsolService -Credential $Credentials

   # Reset password
   $pwd = ConvertTo-SecureString $NewPassword -AsPlainText -Force
   Set-MsolUserPassword -UserPrincipalName $UserPrincipalName -NewPassword $pwd -ForceChangePassword $false
}

Set-Password -UserPrincipalName $UserPrincipalName -NewPassword $NewPassword