There is nothing built-in to do that. You would need to use PHPMyAdmin, or similar interface, to edit the database fields directly. You would change the date of the tasks in the tasks table, and then you would have to edit the date for each corresponding signup in the signup table, making sure to keep the date format the same.
Alternately, you could copy the sheet you set up to a new sheet, edit the tasks the change the dates, and then use the admin feature that allows you to do live search on the sign-up page to sign up all the same people again for the new dates.
Or, once you copy the sheet, use the newly added email volunteers feature to email all the people from the old sheet and have them sign-up again on the new sheet, thus making sure that they are all actually available on the new date.
Or, just use that email feature to notify all of them that the dates have changed and to have them contact you if they can’t make it on the rescheduled dates.
Our reasoning on not allowing you to change dates for things that people already signed up for when I did the plugin for our PTA was that we don’t know if those same people would be available at the new date and time, so we didn’t want the chair people to change dates after people have already signed up. Sure, that feature could be added, but it’s not a quick or simple feature, and not something we needed when I originally programmed this (actually a modification of another existing plugin at the time). You could certainly add that to the feature request section, but something like that would also involve notifying everyone that the dates have changed and somehow having them confirm if they can do the new date/time or not.