iPad OS update

While the requirements will differ slightly between iPad models, iPadOS 16.x when installed requires ~9.2GB for the OS alone. It is generally recommended that you keep at least 4GB of free space available, regardless of iPad model, for the iPad to be able to function with reasonable performance.

You should be to estimate the additional space requirements for your iPad. First, establish the space occupied by the currently installed version of iPadOS:

Settings > General > iPad Storage - scroll to the bottom of the page where you will see the space required (GB) for iPadOS.

Subtract this number from the space required for iPadOS 16.x (i.e., ~9.2GB). This is the approximate additional storage required.

Any file-size information that you see when downloading an available iOS/iPadOS update is for the compressed download only; the download file-size has no relationship whatsoever to the space required for an installed system software update.

A downloaded update must be first be unpacked into available space - and verified - prior to installation on the device. Within limits, insofar as the device is able, your iPad will automatically offload Apps and user-data to free sufficient space to perform system software update - and will restore any offloaded data and Apps upon completion of the update.

Where insufficient space is available on the target device to perform an update, it may be necessary to perform the update using a Windows PC or Mac computer. The computer will download, unpack and verify the update prior to installation upon the target device. This process requires less free-space to be available on the target device.

If using a computer to perform the update, you will need access to either a Windows PC (with iTunes for Windows installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

iPadOS 15.7.6 will invariably require more system storage than iPadOS 13.x - but consider this:

1) As is you cannot install many Apps that require a higher version of iPadOS than is already installed - limiting usefulness and utility of your existing iPad. If necessary, shedding some less used or incompatible Apps may make your iPad more useful than is currently the case.

2) iPadOS 15.7.6 includes vital system security updates for vulnerabilities that are being actively exploited. Update is strongly advised if your privacy and security - and that of your personal and sensitive data - are important to you.

These support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

Be aware that while an iPad may be capable of update, being so far behind the most recent compatible version, you may need to use a computer to perform the update. To do so, you will need access to either a Windows PC (with iTunes for Windows installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

You should note that for your iPad to install an available update, its battery must be charged to at least 50% capacity - and/or be connected to its Power Adapter. Similarly, the iPad can only be updated over a standard WiFi connection - or be updated by a PC or Mac using iTunes or Finder - over a USB connection.

If you have insufficient space to perform the update, you may need to delete unused Apps or unneeded data. 

These support pages should prove to be helpful:

About storage on your device and in iCloud – Apple Support

What's the difference between device storage and iCloud storage? – Apple Support

Manage your iCloud storage – Apple Support

How to check the storage on your iPhone, iPad and iPod touch – Apple Support

Clear Other storage on your iPhone or iPad - Apple Support

“Offloading” Apps can be a helpful method of creating temporary space for a software update.

https://9to5mac.com/2020/01/21/how-to-offload-apps-in-ios-to-save-space-without-deleting-their-data/

Additional information about creating sufficient space for installation of a software update can be found here:

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

External storage options can often help with managing limited internal storage. For iPad models with a Lightning port, to use external USB storage, you’ll need an Apple Lightning to USB3 Camera Adapter:

• Lightning to USB 3 Camera Adapter

https://store.apple.com/uk/xc/product/MK0W2ZM/A