You can do this either by clicking on the 'modifications' link on a desired reservation's page, or you can do this in the reports&schedule tab (day or week view only, this will not work in the month view) you can simply drag and drop the grey square next to a reservation to the target placement. This way you can change the start date/time or also at the same time you can select another resource for the reservation. Once you drop the grey square into a new location you'll still need to confirm the change in a page which will open.