A confirmed objective is one that is no longer in draft and is effectively 'in progress'.
Either add a new objective or click into a draft objective and choose the option at the bottom of the objective pop-up 'Save as confirmed'.
Your objective is no longer in draft and effectively 'in progress'. You can not edit the question fields within the objective as these have now been locked in, however, you can add progress updates.
Note: If you're trying to confirm one of your own objectives and you don't see the option 'Save as confirmed', it means there's a setting on your account that only allows your manager to confirm your objectives.