How to change my previous mobile number to the new one?
I need to update my mobile number. The old number is no longer available. I cannot retrieve all the photos and videos on my previous iCloud account and apple id
iPhone 11, iOS 15
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Hi nevermind11,
Welcome to the Apple Community
Apple Support's article: "Two-factor authentication for Apple ID"
says:
Update your trusted phone number on your Apple ID account page
- Go to appleid.apple.com.
- Sign in with your Apple ID.
- Go to the Security section and click Edit.
If you want to add a phone number, click Add a Trusted Phone Number and enter the phone number. Choose to verify the number with a text message or automated phone call, and click Continue.
To remove a trusted phone number, click 
next to the phone number you want to remove.
The Complete article: https://support.apple.com/en-us/HT204915
Hope this works ok for you.
All the best :-)