I want to add my wife to my Microsoft 365 subscription

Hello

You will need to cancel the personal subscription first, then sign up for the family plan.

If your subscription was done via Microsoft directly, Please go to https://account.live.com

Sign into your Microsoft account. Then go to Services. Click on Manage for the subscription plan you want to cancel. Then chat with support to discuss refund: go to https://support.microsoft.com/contactus, Type "Billing" and press Enter. On the next screen, click "Contact support" at the bottom of the page, then choose "Microsoft 365" > "Manage my subscription". You’ll then see the option to chat with a Microsoft support agent who has access to your account to help review the issue.

If it was done via Apple App Store, you need to cancel within your Apple account, Refund needs to be requested from Apple, not Microsoft, because the payment processed via Apple.

Microsoft explains it here:

https://support.microsoft.com/account-billing/how-to-get-a-refund-on-a-microsoft-subscription-67dca30b-b323-44d5-acc2-e02f9902c472

Apple's refund request instruction is here: https://support.apple.com/118223