How do you delete old number off of iCloud account

See the section - Keep your trusted phone numbers up to date in the article https://support.apple.com/en-in/HT204915#trusted

if you have set up the secondary authentication account , enter with Apple ID and password and security answers for questions , the procedure would be same to edit the trusted number .

Make sure before deleting the old trusted number you must add a new trusted number and get it verified .