If you mean for iCloud keychain, you can change the phone number that receives the SMS verification code by doing the following on an iOS device or Mac that already has your iCloud keychain enabled:
- In iOS 8, tap Settings > iCloud > Keychain > Advanced. In iOS 7, tap Settings > iCloud > Account > Keychain. Make sure the phone number under Verification Number is correct. If not, enter another phone number.
- In OS X Mavericks v10.9, choose Apple () > System Preferences. Click iCloud, then click Account Details. Make sure the phone number under Verification number is correct. If not, enter another phone number.
If you don't have access to a device/Mac that has your iCloud keychain enabled, you'll have to contact Apple support for assistance.