Starting from iOS 6 SKStoreProductViewController class was introduced. You can link an app without leaving your app. Code snippet in Swift 3.x/2.x and Objective-C is here.
A SKStoreProductViewController object presents a store that allows the
user to purchase other media from the App Store. For example, your app
might display the store to allow the user to purchase another app.
Drive Customers Directly to Your App
on the App Store with iTunes Links
With iTunes links you can provide your
customers with an easy way to access
your apps on the App Store directly
from your website or marketing
campaigns. Creating an iTunes link is
simple and can be made to direct
customers to either a single app, all
your apps, or to a specific app with
your company name specified.