Delete a VPC attachment in AWS Transit Gateway
To delete a VPC attachment using the console
-  Open the Amazon VPC console at https://console.aws.amazon.com/vpc/ . 
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            On the navigation pane, choose Transit Gateway Attachments. 
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            Select the VPC attachment. 
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            Choose Actions, Delete transit gateway attachment. 
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            When prompted, enter deleteand choose Delete.
To delete a VPC attachment using the AWS CLI
Use the delete-transit-gateway-vpc-attachment command.