my ipad won’t update?

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 your iPad may be too far behind the current version to be updated directly from iPad settings. As such, you will likely need to use a computer to perform the update. You will need access to either a PC (with iTunes 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.

Error #4000 is not specifically listed by Apple - however, this error is often associated with an internet connection problem, or security software installed on the PC/Mac with which the update is being performed. Temporarily disabling your security/anti-virus software may resolve the issue, however, you do so at your own risk.

There have been many reports of some iPad models having difficulty updating to iPadOS 15.0/15.0.1/15.0.2. Anecdotal reports indicate that for WiFi+Cellular models of iPad that are failing to accept the update, removal of the SIM tray during the update allows the update to complete successfully. Others report success by inserting a SIM (active or not).