Update App Credentials using Powershell

 import-module MSOnline
 $msolcred = get-credential
 connect-msolservice -credential $msolcred

 $clientId = "ac3f9ccc-edee-4ceb-943a-4870758c4264"
 $keys = Get-MsolServicePrincipalCredential -AppPrincipalId $clientId
 Remove-MsolServicePrincipalCredential -KeyIds @("7a5f64f2-93eb-42c6-8ab8-dd25940838dc","0abb7c26-b95e-4be9-9b4e-82b99a8cbd18","165bd912-093e-4aec-a286-221290cb31e7","5e93eb02-29fa-4e74-b5ac-f6d60aedbf03","5e93eb02-29fa-4e74-b5ac-f6d60aedbf03","5e93eb02-29fa-4e74-b5ac-f6d60aedbf03") -AppPrincipalId $clientId

 $bytes = New-Object Byte[] 32
 $rand = [System.Security.Cryptography.RandomNumberGenerator]::Create()
 $rand.GetBytes($bytes)
 $rand.Dispose()
 $newClientSecret = [System.Convert]::ToBase64String($bytes)
 New-MsolServicePrincipalCredential -AppPrincipalId $clientId -Type Symmetric -Usage Sign -Value $newClientSecret -StartDate (Get-Date) -EndDate (Get-Date).AddYears(1)
 New-MsolServicePrincipalCredential -AppPrincipalId $clientId -Type Symmetric -Usage Verify -Value $newClientSecret -StartDate (Get-Date) -EndDate (Get-Date).AddYears(1)
 New-MsolServicePrincipalCredential -AppPrincipalId $clientId -Type Password -Usage Verify -Value $newClientSecret -StartDate (Get-Date) -EndDate (Get-Date).AddYears(1)
 $newClientSecret