How to update my iPhone 7 Plus?

You cannot pick and choose an iOS version. What you get is Settings > General > Software update > [All options are available here]

You need to correctly Identify your iPhone model - Apple Support

These are the iOS options based on an iPhone Model -->

• iOS 15.8 —>
  • Chipset A9 - 6S (2GB RAM) / 6S Plus (2GB RAM) / SE (1st Gen) (2GB RAM)
  • Chipset A10 - 7 (2GB RAM) / 7 Plus (3GB RAM)

Know how to...

• Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
• Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)

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 this is your personal device then you can update your device with the current iOS

Know how to...

• Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
• Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If this is your corporate iPhone, it is important to reach out to the system administrator of your corporate. They will be able to assist you with any issues or concerns related to your device.

However, if it is your personal device, there are a few things to consider. Firstly, check if you have any profiles installed by your corporate, organization, enterprise, or school management. These profiles are typically used to manage certain settings or restrictions on your device.

If you do have a profile installed, and you no longer wish to keep it, you can follow the steps provided by Apple to remove configuration profiles on your iPhone. By doing so, you will eliminate the profile and its associated settings from your device. Install or remove configuration profiles on iPhone - Apple ...

In case you need further guidance or encounter any difficulties during the process, it is recommended to contact the system administrator of the organization that provided the profile to you. They will be able to assist you in removing the profile or addressing any concerns you may have.