You can access an existing reminder and modify its properties in many ways, but in essence - anywhere you see a reminder icon - you can double click on it with your left mouse button or right click on it and select View/Change menu item to view/modify it's properties
This can be done on the main calendar view, the upcoming reminders list, daily reminders list or the search dialog.
NOTE: If you wish to move an event for a few days you can simply drag and drop it on the main calendar view.
Once you have selected the option to modify a reminder the "View/Modify Reminder" dialog will show up and allow you to change appropriate values.