How do I get rid of my lower level ant troops?

You have 2 options. You can upgrade them. Go to the training building and select the lower tier. There will be an up arrow above the image. Tap this and you can upgrade them to the highest tier you have available.

You can dismiss them, this deletes them completely. Again in the training building select the tier of ants you want to remove and you will see red circle with a line icon to the right of the number of ants. This lets you select and dismiss the ants you have.

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