How can I change my iCloud iD

See this Apple article: Change your Apple ID

Change your Apple ID - Apple Support (SG)

Why change Apple ID? 

FYI: you can't merge multiple Apple ID accounts.

See this Apple article: Sign in with your Apple ID

Sign in with your Apple ID - Apple Support

Your Apple ID is the account that you use to access all Apple services and make all of your devices work together seamlessly.

Have you forgotten your Apple ID?, Check your Apple ID via your email box, see this Apple article: If you forgot your Apple ID

If you forgot your Apple ID email address or phone number - Apple Support                  

OR see -> Having trouble signing in?

https://iforgot.apple.com/password/verify/appleid

If you've forgotten your Apple ID email address or phone number

If you've forgotten your Apple ID email address or phone number – Apple Support (UK)

Then see this Apple article: What to do after you change your Apple ID or password

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