Most fields of the bundle are locked after creating it. If you want to change the games in the bundle you will generally need to create a new bundle. In some special cases itch.io staff may assist in edits to the bundle, but this is generally for larger bundles that have already been created/published and there are specific needs about changing the lineup.

From https://itch.io/docs/creators/bundles#getting-started/creating-the-bundle

Why are some fields locked? — During the approval period, we don’t want participants to have any surprises about the promotion they are about to join. In order for the participant to confidently decide to join the bundle they need to see the final arrangement of the promotion. itch.io staff may be able to make some changes to the bundle, but it may be easier to re-create the bundle if substantial changes need to be made.