I can't update my iPhone 6s Plus to iOS 15.7.3

15.7.3 is the highest version available for your 7 year old phone. If you are asking how to update TO 15.7.3 go to Settings/General/Software Update and follow the instructions there.

To update over the air from the phone all of these requirements must be met:

• You need at least 6 GB free for iOS 15.x; more is better
• You must be connected to a Wi-Fi network (and not a personal hotspot)
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles
• The phone cannot be in Airplane mode and must have a cellular data connection
• You cannot still have a beta version installed on the phone
• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it

If you can't meet all of them you can still update using your computer.