I have two apple accounts. One for my iPhone and one for my iPad. How can I change to one for both

You should be mindful that any App purchases are forever tied to the AppleID from where the App purchase was made - and cannot be transferred to a different AppleID account. This may influence your choice of which AppleID you intend to use moving forwards.

Having chosen which AppleID you wish to keep, you will need to sign-out of the other device.

On the device that you intend to change the AppleID, begin by disabling the Find My iPad service. This step is essential to eliminate the Activation Lock that may otherwise cause future difficulties:

Settings > [Your Name / AppleID] > Find My > Find My iPad - set to OFF

Having disabled Find My iPad, you can now sign-out of iCloud:

Settings > [Your Name / AppleID] > Sign Out

When you sign-out, you may be offered the opportunity to save a local copy of your data. Consider carefully the implications; if you chose to do so (most likely the most appropriate option), the iCloud data associated with the current account will eventually be merged with the AppleID account that you intend to use later. By contrast, not saving a local copy will not merge current iCloud data.

After you sign-out from iCloud, a restart of your iPad is iPad is recommended. A simple shut-down and restart will suffice.

After restarting your iPad, sign-in to your preferred AppleID account. If a local copy of your data was previously saved during the previous sign-out procedure, this data will be automatically merged with the iCloud data of the preferred AppleID - without duplication.

Mygreatgrandchildren3 wrote:

Can I have one apple account for both iPhone and iPad ?

Yes you can.

But you have to delete the account on the device you don't want.

FYI: Content from one Apple ID cannot be merged, associated or transferred to another Apple ID.