Welcome to the Apple Community.
In order to change your Apple ID or password for your iCloud account on your iOS device, you need to delete the account from your iOS device first, then add it back using your updated details. (Settings > iCloud, scroll down and hit "Delete Account")
Providing you are simply updating your existing details and not changing to another account, when you delete your account, all the data that is synced with iCloud will also be deleted from the device (but not from iCloud), but will be synced back to your device when you login again.
In order to change your Apple ID or password for your iCloud account on your computer, you need to sign out of the account from your computer first, then sign back in using your updated details. (System Preferences > iCloud, click the sign out button)
In order to change your Apple ID or password for your iTunes account on your iOS device, you need to sign out from your iOS device first, then sign back in using your updated details. (Settings > store, scroll down and tap your ID)
I have followed your advice and changed my Apple ID on my IPAD. However, when I try to log in for updates etc to Apps the old ID came up or a message that says my ID has been disabled. I had no end of trouble trying to re-set my password multiple times (on my old ID), but didn't know that Apple doesn't work with Google Chrome. In the end I have created a new ID, but now still can't download iPAD Apps with the new one.
I don;t know why this is soooo difficult. What do I need to do next?
Thanks, in anticipation