Similar to adding features, you can remove features from a bundle as long as it hasn't been promoted to a downstream environment. To do so, select the bundle, click the 'more' icon and select 'Remove features':
Select which features you'd like to remove:
Click 'Remove features' to confirm.
Note: You can only remove the most recently added features in order. It's not yet possible to remove an arbitrary feature from elsewhere in the bundle.
Once you remove features, the removed features will be reopened against the target environment, just as they would if you'd abandoned the bundle.