If you've deleted an app by mistake or simply want to reinstall it, here's how to get your apps back on your MacBook Air.
There are two ways to do this: via the App Store or using Time Machine.
App Store
Open the App Store on your MacBook Air and click on the Purchases tab. You should see a list of all the apps you've ever downloaded (including those that have since been deleted). Find the app you want to install and click Download. The app will now be downloaded and installed on your machine.
Time Machine
If you have a Time Machine backup of your machine from before you deleted the app, you can restore it using that backup. To do so, open Time Machine and select the backup from which you want to restore. Find the app in question in the list of files and click Restore. The app will be restored to your machine.