Sharepoint List Items values with Graph API

It's unclear whether you want to (1) Get Data Fields only and Update Data Fields only or (2) you need to Update Data Fields from Selection - when in doubt I'll write both approaches. If your requirement is item (2), you may want to map/model the business rule to develop the routine. According to Microsoft Docs, the following examples:

(1) Get Data Fields only:

GraphServiceClient graphClient = new GraphServiceClient( authProvider );

var queryOptions = new List<QueryOption>()
{
    new QueryOption("expand", "fields(select=Field1,Field2,Field3)")
};

var items = await graphClient.Sites["{site-id}"].Lists["{list-id}"].Items
    .Request( queryOptions )
    .GetAsync();

(1) Update Data Fields only:

GraphServiceClient graphClient = new GraphServiceClient( authProvider );

var fieldValueSet = new FieldValueSet
{
    AdditionalData = new Dictionary<string, object>()
    {
        {"Field1", "Test1"},
        {"Field2", "Test2"}
    }
};

await graphClient.Sites["{site-id}"].Lists["{list-id}"].Items["{listItem-id}"].Fields
    .Request()
    .UpdateAsync(fieldValueSet);

(2) Update Data Fields from Selection example:

GraphServiceClient graphClient = new GraphServiceClient( authProvider );
    
    var queryOptions = new List<QueryOption>()
    {
        new QueryOption("expand", "fields(select=Field1,Field2,Field3)")
    };
    
    var items = await graphClient.Sites["{site-id}"].Lists["{list-id}"].Items
        .Request( queryOptions )
        .GetAsync();
foreach(var item in items)
{
    //an example of Business Rule: Update item based by some Select into current item
    if(null != item.Fields.AdditionalData["Field1"] && !string.IsNullOrWhiteSpace(item.Fields.AdditionalData["Field1"].ToString()))
    {
        var fieldValueSet = new FieldValueSet
        {
            AdditionalData = new Dictionary<string, object>()
            {
                {"Field2", "Test2"},
                {"Field3", "Test3"}
            }
        };
        await graphClient.Sites["{site-id}"].Lists["{list-id}"].Items[item.Id.ToString()].Fields
        .Request()
        .UpdateAsync(fieldValueSet);
    }
}