How can I change my Apple ID on my iPad

I presume you are attempting to update your AppleID with a new primary email address - this being used to identify the AppleID account - or change your AppleID account Password:

Change your Apple ID - Apple Support

Change your Apple ID password – Apple Support

What to do after you change your Apple ID or password - Apple Support

If you instead wish to use an entirely different AppleID account, you must first sign-out of all Apple Services (such as App Store, FaceTime, Messages, Music, AppleTV etc) and iCloud - after which you can sign-in with an entirely different account. The easiest method to prepare an iPad to use a different AppleID account is to follow this process:

What to do before you sell, give away or trade in your iPhone, iPad or iPod touch – Apple Support