how do i update my phone through icloud.com

You cannot update your iPhone through iCloud.com.

Your iPhone needs to be updated directly on the device here:

Settings → General → Software Update

You can see how to update from your iPhone here:

Update your iPhone or iPad - Apple Support

You can also update from a PC with iTunes, or a Mac, see the articles below:

• Mac: Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

• PC (iTunes): Update your iPhone, iPad, or iPod touch with iTunes - Apple Support