apple ID
I have 2 different apple IDs on 2 devices (iMac and iPhone) How do I change my apple ID on my iMac?
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You can't make a change that will combine/merge the Apple IDs.
Still, see Change your Apple ID https://support.apple.com/en-us/HT202667.
And, here is What to do after you change your Apple ID or
Password https://support.apple.com/en-us/HT204071 (supplementary to what the fist link says)
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See How to create a new Apple ID https://support.apple.com/en-us/HT204316 and
How to switch Apple IDs on your iPhone or iPad
https://www.imore.com/how-change-apple-id-your-iphone-or-ipad
See the data implications below.
Some of these things may apply even though not changing an Apple ID: What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071
Keep in mind that some things available under a first Apple ID will not be available under a new Apple ID. For example, you won’t be able to transfer anything purchased from the App and iTunes Stores to the new ID. And, there are other issues to be addressed.
And see what Eric Root says on this matter here https://discussions.apple.com/thread/251483565 and
what Winston Churchill says about it here: https://discussions.apple.com/thread/7452747
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But, you can set up family sharing between the IDs to share purchases made with them. See:
Family Sharing https://support.apple.com/en-us/HT201060
Set up Family Sharing https://support.apple.com/en-us/HT201088