I've changed my apple ID at apple.id.com, how is it updated on my devices (iphone and ipad)?

Having updated your AppleID credentials, you need to follow this advice:

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

If the devices can connect to the Internet the change should be transmitted to them. It may help to restart them so they take a fresh look at settings.

Read about "what to do after [or before] you change your Apple ID email address or password --> What to do after you change your Apple ID or password - Apple Support