How do u update the iPad it won’t let me update the iPad when I hit update now

The tags appended to your post suggest that you have an iPad Air2 with iPadOS13 installed. An iPad Air is capable of update to iPadOS 15.7.1 - but does not support iPadOS16.

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 the iPad may be capable of update, it may be too far behind the current version to be updated directly from iPad settings. In such circumstances you will likely 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

If you have a beta profile installed, ensure this has been removed from your iPad - as, otherwise, installation of production software updates may be inhibited. This support page provides the necessary information to remove the beta from your iPad:

How to uninstall iOS beta software - 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.

It is a little hard to help/advise you when you provide little to NO useful information about your iPad.

Here are my questions to you.

Can you, please, confirm what year and model iPad do you have/own?

The “A” model number can be found on the aluminium back of your iPad, in very small print, right under the larger bold print “iPad” on the back of the iPad.

AND/OR

Supply the “M” model number, which can be found in the Settings app, under the General settings panel, then tap About.

Your iPad maybe too old.

What iOS/iPadOS version is this iPad, currently, running? Your iOS/iPadOS version maybe too far behind in iOS/iPadOS versions OR cannot be upgraded/updates any farther than what your iPad OS version is already, currently, running.

Again, your iPad maybe too old.

What is the rated, internal data storage space/capacity of your iPad model and how much of this free, internal data storage capacity is left inside of your iPad model?

All of that additional info can, also, be found in the Settings app, General settings panel, About.

Your iPad needs sufficient free internal data storage space. At the very least, 7-8 GBs or greater internal, free data storage space.

Is your iPad using a VPN? You need to, temporarily, disable the VPN.

Is your iPad running an iPadOS Beta version? You need to remove/uninstall the iPadOS Beta software before you will be able to install the Apple “officially” released version of an iPadOS upgrade/update.

Also, make sure your iPad is charged to, at least, 50%, or greater, before attempting the iPadOS upgrade/update.