How to update software on my iPhone?

As Allan Jones mentioned, we need more information before we can help. In general, you update an iPhone by selecting Settings > General > Software Update. If your phone is eligible for an update, you will be prompted to do so. What happens when you try this? We will also need to know the following:

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. 

Welcome!

Please see this, and then give us enough details to help you:

Writing an effective Apple Support Commun… - Apple Community

We are other end users, not Apple, and can neither see nor touch your tech devices. We can only help whe you provide details.

Please do not make us guess, as that seldom ends well.