In order to delete a Microsoft guest account, you should first create a system restore point. This can be used in case, you end up deleting some of your personal files. Once done, open Settings and go to Accounts. Navigate to Family and under Other People look for the account you want to delete. After finding that account, click on it and select Remove. This way, you will be able to delete a Microsoft guest account pretty easily. Afterward, if you want, you can easily create a new guest account.