In order to edit Scheduled transactions, please first switch to Scheduled transactions section: click on the calendar icon in the menu.
Please note that it is not possible to edit a Scheduled transaction simply by editing the transaction it produces. Scheduled transactions are like templates that create a copy of themselves when the time is due. Once the copy is produced, the created transaction is not aware that it was created by a Scheduled transaction and therefore is completely separate from it.
Once you've switched to Scheduled transactions section, notice that your transactions are listed as little squares below the calendar. To edit one of the scheduled transactions, left-click on it once so that the contextual menu can appear, from which choose Edit. You'll be presented with a transaction window, where you can edit the properties of this transaction.
There is one important thing to note: changes you make here will apply to the next and all future occurrences of the transaction. If you want to make changes to just the next occurrence only, please click on the Scheduled transaction box and select Pay instead. Please note that the transaction will be executed after you save your changes so you might want to wait until the transaction is actually due before you do this.