Question: How to remove retention tags via PowerShell?

Please first run the commands in the article "Install and Configure Windows PowerShell": http://help.outlook.com/en-us/140/Cc952756.aspx

Then run the commands in the following article again.
http://help.outlook.com/en-us/140/cc952755.aspx
 
If it connects to Exchange Online service successfully, you will get the following page:

Then you can run related commands for your requirements.

1. Removes all retention policy Tags
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Enable-OrganizationCustomization

Get-RetentionPolicyTag | Remove-RetentionPolicyTag
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2. Removes retention policy tags from a retention policy
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Enable-OrganizationCustomization

Set-RetentionPolicy "<policy name>" -RetentionPolicyTagLinks $null
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References:

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Set Up and Manage Retention Policies in Exchange Online
http://help.outlook.com/en-us/140/gg271153.aspx?sl=1