The first marijuana dispensary to get approval in Watertown could open its doors in the next month, or so, selling medical marijuana first and several months later for recreational use. The Planning Board heard from Natural Selections on Wednesday night, which is seeking changes to the special permit it received in 2017. The company plans to open the facility in a building in the rear of the property at 23 Elm St., just off Arsenal Street and across from the Target parking lot. The changes are needed to get ready for sales of adult-use marijuana, also referred to as recreational. Before it can open, Natural Selections must get approval from the state Cannabis Control Commission (CCC), said Chief Operating Officer Aidan O’Donovan.