Apart from App store, there's a big world of apps out there. Not all the apps outside app store are reliable, and not all are risky. It depends on which app you are installing and what is the source of app. You may find some very decent open source apps over internet.
To install open source app on your iPhone having iOS 9, you'll need to have Xcode 7 installed on your Mac. Xcode 7 is a developer tool to deploy apps directly to iPhone. Once again, make sure you are deploying an app from a reliable source.
Assuming that you have the app source with you, and wish to deploy it on your iPhone running iOS 9, now we can move forward. Just check the guide given below and follow the steps accurately.
How to Sideload Apps on iPhone and iPad Running iOS 9 using Xcode 7
Step #1. Very first thing is to install Xcode 7 on you Mac; the setup is pretty big and might take few hours. So, it is good if we keep it in downloading while we are busy doing rest of the homework.
Step #2. Create a Developer account from http://developer.apple.com. If you already have a developer account you can skip this step. If not, visit the link and click on “Member Center”. Click on “Create” if you do not have an Apple ID; or enter your Apple ID and password to login.
Step #3. Now open Xcode 7 on your Mac; go to Xcode → Preferences → Accounts. Click on the “+” icon and add your developer Apple account you created in Step 2.
Step #5. Copy the URL from the right side as shown in the screenshot given below.
Step #6. Now on Xcode 7 click on Source Control → Check Out.
Step #7. In the repository location box enter the URL of the app as explained in above screenshot. Now click Next and followed by clicking Download button. The code will now be downloaded in Xcode 7 on your Mac. Once the download is completed you will notice app name along with Ready signal.
Step #8. Connect your iPhone running iOS 9 to your Mac using USB cable. Select your device from the drop down box next to the app name. It may take a while to process the symbol files needed for your connected device.
Step #9. You are ready to deploy app, as soon as you see Ready. Click the “Play” button to start deployment process. The time required depends on the size and build of the app, so have some patience. Make sure your USB connection with your device is not disturbed.
Step #10. Once the deployment process is finished, you will see the app icon on your iPhone home screen. You will need to authorize the app as trusted app, to do this, go to Settings → General → Profile → Your App → and tap Trust.
That's it; you can now use the app on your device running iOS 9. I might sound repetitive, but again a word of advice, do not download any app that is not from a reliable source. You might end up with a dead device or some serious hardware malfunctioning.