To update the value of a cookie, just assign a new value to the cookie name:
document.cookie = 'name=Flavio2'
Similar to updating the value, to update the expiration date, reassign the value with a new expires or max-age property:
document.cookie = 'name=Flavio; max-age=31536000' //expires in 1 year
Just remember to also add any additional parameters you added in the first place, like path or domain.