how to update multi select option set field in d365 using power shell

While using your code also same issue
FaultException`1 - Update : Updating account : 265bcef3-5aeb-ee11-a203-000d3a3b5473 |=> Incorrect attribute value type System.Object[]
Incorrect attribute value type System.Object[] => Incorrect attribute value type System.Object[] => System.ArgumentException: Incorrect attribute 
value type System.Object[]
Parameter name: MultiSelection[TerminalFailure] Failed to Execute Command - Update : RequestID=09d2c983-a780-4458-b806-23eb7bcc1e96 : 
Updating account : 265bcef3-5aeb-ee11-a203-000d3a3b5473 duration=00:00:01.3538665 ExceptionMessage = Incorrect attribute value type System.Object[]
Incorrect attribute value type System.Object[] => Incorrect attribute value type System.Object[] => System.ArgumentException: Incorrect attribute 
value type System.Object[]

To update a multi-select option set field in Dynamics 365 using PowerShell, you first need to retrieve the existing values of the record, modify the multi-select option set field, and then update the record with the new values. Here's how you can do it:

# Connect to Dynamics 365 (you need to define $conn first)

# Define the ID of the account record you want to update
$accountId = "Your_Account_Id_Goes_Here"

# Retrieve the existing values of the multi-select option set field from the account record
$existingOptions = Get-CrmRecord -conn $conn -EntityLogicalName account -Id $accountId -Fields "new_multiselectfield"

# Define the values for the multi-select picklist attribute
$optionSetValues = @(484800005, 484800001, 484800003)

# Create an array to hold new option set values
$newOptionSetValues = @()

# Loop through the values and convert them to CRM option set values
foreach ($value in $optionSetValues) {
    $optionSetValue = New-CrmOptionSetValue -Value $value
    $newOptionSetValues += $optionSetValue
}

# Combine existing options and new options
$newOptions = $existingOptions + $newOptionSetValues

# Create account data
$account = @{
    "new_multiselectfield" = $newOptions  # Multi-Select Option Set
}

# Update the account record with the new multi-select option set values
$accountRecord = Set-CrmRecord -conn $conn -EntityLogicalName account -Id $accountId -Fields $account

In this script:

1. Connect to your Dynamics 365 instance. Make sure you have defined the $conn variable properly with connection details.
2. Retrieve the existing values of the multi-select option set field from the account record using Get-CrmRecord.
3. Define the new values for the multi-select option set field.
4. Convert the new values into CRM option set values using New-CrmOptionSetValue.
5. Combine the existing options and new options into a single array.
6. Create an account data array with the updated multi-select option set field.
7. Update the account record with the new multi-select option set values using Set-CrmRecord.

Make sure to replace "new_multiselectfield" with the actual schema name of your multi-select option set field in Dynamics 365. Also, replace "Your_Account_Id_Goes_Here" with the ID of the account record you want to update.