If anybody is looking to update a relationship with a user here is my working code

General
1

class func updateRetailers(userID:String) {

    let whereClause = "objectId='\(userID)'"

    let dataQuery = BackendlessDataQuery()

    dataQuery.whereClause = whereClause

    backendless.persistenceService.of(BackendlessUser.ofClass()).find(dataQuery, response: { (collection) in

        let currentPage = collection.getCurrentPage()

        let user = currentPage[0] as! BackendlessUser

        var newRetailers = [RetailerPage]()

        var newRetailer = RetailerPage()

        newRetailer.retailerName = "THIS IS GOOD"

        newRetailers.append(newRetailer)

        let retailers = user.getProperty("followedRetailers") as! [RetailerPage]

        for retaila in retailers {

            newRetailers.append(retaila)

        }

        user.setProperty("followedRetailers", object: newRetailers)

        backendless.userService.update(user)

        

        

        }) { (fault) in

            print("NO FAULTS")

    }

}
2

This may be:

    func addRetailersForUser(userID:String, addRetailers:[RetailerPage]) {
        
        backendless.userService.findById(userID, response: { (user : BackendlessUser!) in
            
            var retailers = user.getProperty("followedRetailers") as! [RetailerPage]
            for retailer in addRetailers {
                retailers.append(retailer)
            }
            
            user.setProperty("followedRetailers", object: retailers)
            self.backendless.userService.update(user)
            
        }) { (fault) in
            print("NO FAULTS")
        }
    }