when I want to verify my keychain they send SMS to a phone number that i stopped using, how can I change this phone number to my new one?

If you didn't receive the verification code via SMS

1. Make sure that you have a strong cellular network connection on your phone.
2. Make sure that your phone number can get SMS messages. To check, ask someone to send you a text message.
3. Make sure that the correct phone number is associated with your account:
   • On your iPhone, iPad, or iPod touch, 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.
   • On your Mac, choose Apple menu > System Preferences. Click iCloud, then click Options next to Keychain. (In OS X Mavericks or earlier, click iCloud, then click Account Details.) Make sure the phone number under Verification number is correct. If not, enter another phone number.

If you can't access a device that has iCloud Keychain enabled, contact Apple Support and verify your identity to get help setting up iCloud Keychain.

From Get help using iCloud Keychain - Apple Support