how do I change the phone number that my keychain verification code is sent to? I no longer have access to that number

You can change it on an iOS device or Mac that already has iCloud keychain set up on it as follows:

• Devices using iOS 7.0.3 or later: Tap Settings > iCloud > Account, then tap Keychain. Check that the phone number listed under Verification Number is correct. If not, enter another phone number.
• Macs using OS X Mavericks v10.9 or later: Choose Apple () menu > System Preferences. Click iCloud, then click Account Details. Check that the phone number listed under Verification number is correct. If not, enter another phone number.