The following is the official link for deleting your Facebook account, just click the link and confirm your action, then your account will be completely deleted after two weeks:
REPORTING THE ACCOUNT OF DECEASED PERSON
If you have lost your loved one, you can contact Facebook so that they can delete that account, or they can memorize it.
To request the removal of your loved ones account, read this help:
And if you would like to memorize the account of your loved ones, click the below link to request: