How to handle an update of list that have new records and and old updated records in the same list

I have an application with users, and in the UI there is a profile page where you can manage the

user personal Info.

In the profile page each user have a list of contacts.

In the back-end I give the client the users as list of user object like:

[{
    userId: 1,
    contactId: 3,
    name: "John",
    email: "john$gmail.com"
},
{
    userId: 1,
    contactId: 4,
    name: "Mary",
    email: "mary$gmail.com"
}]

Now when he sends me update request I'm trying to figure out what is the best way to send the data from the client, and how should I manage this case cause he can send me client that is already in the db and one that is not. how would you go about that?

lets say now he wants to send me:

[{
    userId: 1,
    contactId: 3,
    name: "Mark",
    email: "mark$gmail.com"
},
{
    userId: 1,
    contactId: 4,
    name: "Mary",
    email: "mary$gmail.com"
},
{
    userId: 1,
    name: "Mary",
    email: "mary$gmail.com"
}]

So the first one he edited, the second as it was and he added one that is new, but he doesn't have an id so I don't know how to update this scenario the correct way..

appreciate any help :)