I want to change the name that comes up it was my Dad's computer and he has since died, it upsets my Mum seeing his name.
I dont want to delete the account.Then you need to create a new account for a different user.
[Edit]
Sorry, my original reply was a bit brief. Your Dad's account was likely an Administrator's account and you should probably create a new Administrator account to replace it. If you then want to delete your Dad's original account you may lose access to any of his documents that you want to keep so you should back those up first.
See here;
https://www.iseepassword.com/blog/how-to-create-a-new-administrator-account-in-windows-10/
https://www.iseepassword.com/blog/delete-the-administrator-account-in-windows-10/