It should appear in your app drawer, usually labeled as 'Camera'.
Even if you download another app by the same name, both should appear.
If you want the downloaded app to stop being opened all the time by other apps, then you need to clear the defaults for that app.
Settings
Apps (show all)
(whatever camera app it is)
Open by default
Clear defaults
The next time that an app opens the camera, you will be asked which camera app to use.
The time after that, you will be asked again, and then given the option to make your choice permanent (setting the default) or just once.
If you choose 'just once', then you will be asked again each time an app wants to open a camera app.
You can stop this altogether by having only one camera app enabled (ie. disable the stock camera app, or uninstall all other camera apps) on your device.