When you set up your Apple ID, you also set up an iCloud account that was linked to that Apple ID, correct?
If the Apple ID is not an @me or @mac email address, you can change your Apple ID email address, which will change the email portion of your @icloud.com address.
So, if your Apple ID is Mary@gmail.com, and your iCloud address is Mary@iCloud.com, and you want to change your Apple ID to Mary123@gmail.com, then that will also change the iCloud account associated with that Apple ID to Mary122@iCloud.com.
To do this go to Manage Your Apple ID, sign on and EDIT your Apple ID to the new email address you want to have.
BEFORE DOING THIS: sign out of iCloud, iTunes & App Store, FaceTime, Messages, etc. on your devices and computer.
Cheers,
GB