@lchutny So you opened the workout but didn’t start it and accidentally checked workout done? I have done that - it can’t be reversed unfortunately but what you can do is go to the videos tab and start the workout from there. You will have two entries in the calendar - one which you can just ignore.
However, if instead you started the workout and then hit complete, you may be able to delete the workout and if it is part of your plan it will revert to incomplete status.
Hope that helps! I think they are working on a new calendar to fix some of these issues but it is part of a new platform so it may be a while.