No, allowing contacts to change event selections themselves is, frustratingly, a missing feature. If you're interested in helping to implement this, either with contributions of code or funding, I'm sure others would be interested in working with you on this (I know it's a feature that I wish we had).
If it's just a matter of adding more options and not removing, as long as you have "Allow same email and multiple registrations?" checked, they can register again to add the additional options. The downside there is you can't merge the two event registrations on the backend. For the other participants, they can just register again for them, though that's less than ideal as well.
You can, of course, have people get in touch and then you can edit their registrations on the back end and charge them the additional amount required there.
Welcome to CiviCRM, by the way.