Two Options to undo a recon in QBO:
1. open the register and double click on the R to remove it, one transaction at a time

2. if you have an accountant who signs in to your QBO account, have him log in and use the accountant tool, undo recon
or
invite yourself as the accountant, create the log in, and log in as the accountant so you can use the accountant tools (use a different email for the invite)