Please how can I update my iPhone

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

If your phone is qualified for an update and you have a good Wi-Fi Internet connection....and....the battery is fully charged......

Tap Settings > General > Software Update

If an update is available, it will appear and you can tap to proceed.

If a message appears to tell you that you already have the latest software.....no updates are available.