To update my phone for 15

Please tell us:

Which iPhone do you have?  

To find out, select Settings > General > About. Look for Model Name. 

What is its iOS? 

To see this, select Settings > General > Software Update. That screen will identify your iOS version. 

Also, which app is not allowing you to download? What happens when you try?

This may help --> Apps that require higher iOS versions than your device can be updated to... - Apple Community