How to update my iPadOS from 15.8.3 to 16.0?

The tags appended to your post suggest that you have an iPad mini4. The mini4 is capable of update to iPadOS 15.8.x; the mini4 fundamentally does not support iPadOS16 or later

Whether or not you are able to update any iPad to a newer version of iOS/iPadOS will be dictated by the iPad model - and the currently installed version of iOS. You can find this information here:

Settings > General > About

Not sure which iPad you have? Identify your iPad model.

Here is a list of the highest version of iOS supported by the older models of iPad (for simplicity - “generations” are shortened to a number) that are restricted in their ability to be updated:

32-bit devices (obsolete)

iPad1 - iOS 5.1.1

iPad2, iPad3 and iPad mini1 - iOS 9.3.5 (WiFi Only models), iOS 9.3.6 (WiFi+Cellular)

iPad4 - iOS 10.3.3 (WiFi Only), iOS 10.3.4 (WiFi+Cellular)

64-bit devices

iPad Air1*, iPad mini2 and iPad mini3* - iOS 12.5.7

iPad Air2, iPad mini4 - iPadOS 15.8.3

iPad5, iPad Pro (9.7"/12.9" 1st generation)* - iPadOS 16.7.10

iPad6, iPad Pro (10.5"/12.9" 2nd generation) - iPadOS 17.7.3

(*) obsolete

Newer models are all able to be updated to the current version - iPadOS 18.2

• iPad Pro 13" (M4)
• iPad Pro 12.9" (3rd generation and later)
• iPad Pro 11" (1st generation and later + M4)
• iPad Air (3rd generation and later + M2)
• iPad (7th generation and later)
• iPad mini (5th generation and later + A17 Pro)

Whilst many App titles remain compatible with your iPad, if you want or need to access to Apps that require a more recent version of iPadOD than your iPad is capable, your only option is replace your iPad with a much newer or current model.

Your iPad Mini 4 no longer receives software updates for the latest iPadOS operating software, and cannot be updated past iPadOS 15.8.3.

An iPad Air 2 or iPad mini 4 isn't capable of 16 or above; they require an A9 or better CPU.

(257262)

Unfortunately your iPad is no longer supported and unable to receive any more system updates higher then iOS 15 due to its age. If you wish to get access to to the new up to date software you will need to get a newer device.