You cannot change the connection in an existing Google or Microsoft ad generator directly, but you can change the connection when making a copy of an existing ad generator. To do this, go to the Settings of the Ad Generator you want to change the connection for. Click on the gear icon in the top right and then 'copy ad generator':
You will now see a screen where you can configure the copy of the ad generator. If you want to change the connection, do this in the 'With connection' option:
Configure the name and campaign name for the copy, and click on 'Copy ad generator'. A copy of the existing ad generator will appear with the changed connection. Now simply deactivate the 'old' generator and activate the copied generator, and you have effectively changed the connection in your ad generator.