Had the same issue; found this guy on https://youtu.be/G1HgijRQv00?feature=shared youtube and this worked.  I haven't watched this is a while, but he basically explains how you can create another account using a different email, then invite that email address to be an "accountant".  Once you are in "accountant mode" you have an undo reconciliation button, just like we had for 20 years in the desktop version. This solution worked for me.  With that said, I hate this online version and am considering on trying to switch to Patriot, but haven't tried it yet, as the thought of switch scares me almost as much as the thought of continuing to use this QBO.  If anyone has any experience with Patriot, please comment.

In case the link I posted above gets zapped when I post this, the guys name on youtube is Hector Garcia CPA.