Trying to sync data from third party api

If this were my project this is how I would break down the tasks, in this order.

First, start an empty console application.

Next, write a method that gets the list of users from the API. You didn't tell us anything at all about the API, so here is a dummy example that uses an HTTP client.

public async Task<List<User>> GetUsers()
{
    var client = new HttpClient();
    var response = await client.GetAsync("https://SomeApi.com/Users");
    var users = await ParseResponse(response);
    return users.ToList();
}

Test the above (e.g. write a little shoestring code to run it and dump the results, or something) to ensure that it works independently. You want to make sure it is solid before moving on.

Next, create a temporary table (or tables) that matches the schema of the data objects that are returned from the API. For now you will just want to store it exactly the way you retrieve it.

Next, write some code to insert records into the table(s). Again, test this independently, and review the data in the table to make sure it all worked correctly. It might look a little like this:

public async Task InsertUser(User user)
{
    using (var conn = new SqlConnection(Configuration.ConnectionString))
    {
        var cmd = new SqlCommand();
        //etc.
        await cmd.ExecuteNonQueryAsync();
    }
}

Once you know how to pull the data and store it, you can finish the code to extract the data from the API and insert it. It might look a little like this:

public async Task DoTheMigration()
{
    var users = await GetUsers();
    var tasks = users.Select
    (
        u => InsertUser(u)
    );
    await Task.WhenAll(tasks.ToArray());
}

As a final step, write a series of stored procedures or a DTS package to move the data from the temp tables to their final resting place. If you are using MS Access, you can write a series of queries and execute them in order with some VBA. At a high level it would:

1. Check for any records that exist in the temp table but not in the final table and insert them into the final table.

2. Check for any records that exist in the final table but not the temp table and remove them or mark them as deleted.

3. Check for any records in common that have different column values and update the final table.

Each of these development activities raises it own set of questions, of course, which you can post back to StackOverflow with details. As it is your question doesn't have enough specificity for a more in-depth answer.