Follow the instructions posted by LCJim but instead of right clicking and selecting "Edit" you can right click on the transactions and select "Reconcile" -> "Not Reconciled". You'll receive a popup window asking "Mark currently selected transactions not reconciled?" and click the "Yes" button. The previously selected transactions will now be marked as unreconciled.
Unfortunately there isn't an un-reconcile feature. Best option is to restore from a backup, Quicken automatically backs up your datafile every week, if you go to file/open there is a BACKUP directory, all caps, in there and you can open it and look for the most recent backup before the reconcile.
The other option, and it has it's problems, is to do a multiple select on the transactions in the register, Click on the last transaction you want to unreconcile, then hold down shift and go back to the earliest transaction you want to unreconcile and click on it, or do a CTRL-Click to select individual transactions.
Right mouse and select EDIT, you will get the find/replace window open, at the bottom you can get a Replace "Cleared Status" with and you can do a 'c' or blank to unreconcile it.