Dear Rattanjot Kaur
Have a nice day! Thank you for posting to the Microsoft community. We will be happy to help you!
First of all, we regret for the inconvenience caused to your work. Based on your description, I have a general understanding of your question,if you mean you can't login your account because of losing verification code, then please kindly refer to following information Troubleshooting verification code issues - Microsoft Support
In addition, if your problem persists and you are unable to change your number because you do not have access to your account, then our forums lack the relevant permissions and tools, so in order to better help you and not waste more of your time, I strongly recommend that you contact the 365 support team, please refer to the following details Customer service phone numbers - Microsoft Support
By the way, after you contact support, please kindly check following information about how to change phone number, for your reference
- Change the email address or phone number for your Microsoft account - Microsoft Support
- If you are using admin account, please kindly check Update your admin phone number and email address - Microsoft 365 admin | Microsoft Learn
Also, if you still need further help, I still double confirm more detaails about your issue such as your type of account(business account within business subscription/outlook.com/hotmail.com/live.com/msn.com/imap account/pop account)
Thank you in advance for your understanding. Your patience and cooperation will be greatly appreciated. Hope for all the best!
Sincerely
Kerry Chen | Community Moderator