That depends on which version of Microsoft 365 you already have. Following are the different options.
I have an existing Microsoft 365 Personal subscription
If your existing Microsoft 365 Personal subscription is not set to automatically renew, you get the free offer. Your existing Microsoft 365 Personal subscription will be extended by the duration of the Skype calling subscription (1, 3 or 12 months).
If your existing Microsoft 365 Personal subscription is set to automatically renew, you cannot claim the free offer unless you cancel your existing Microsoft 365 Personal subscription.
I have an existing Microsoft 365 Home subscription
You will need to cancel your existing Microsoft 365 Home subscription if you want to claim the free offer. After cancelation, if your existing Microsoft 365 Home subscription was not set to automatically renew, the free Microsoft 365 Personal subscription will start at the next Skype subscription renewal date.
I bought Microsoft 365 Personal after buying a Skype subscription with free Microsoft 365 Personal
You can still get your free subscription. The expiry or renewal date of your non-Skype Microsoft 365 Personal subscription will be extended by 1, 3 or 12 months.
I bought Microsoft 365 Home after buying a Skype subscription with free Microsoft 365 Personal
You will be upgraded to Microsoft 365 Home, but you won’t get the free offer at the next Skype subscription renewal date unless you cancel your Microsoft 365 Home subscription.