Like others have said, you should await the async call. This ensures that the action is executed in a predictable fashion. The intend is to execute it right now and to wait for the result of the action.
That being said, the discord documentation describes this method as follows:
public Task ModifyMessageAsync(ulong messageId, Action<WebhookMessageProperties> func, RequestOptions options = null)
The second parameter describes a delegate based on WebhookMessageProperties. It can easily be defined by a lambda, like such:
x => { }
Now bear in mind the x is arbitrary, you can chose whatever designation you like, even whole words, I just kept it short for the example.
Between the bracelets, you can use access any properties that are in the WebhookMessageProperties class, by using x.SomeProperty. Where SomeProperty has to be a known property of that class if that makes sense.
One of the known properties is for example:
string Content { get; set; }
So here is how you can use a lambda to change the Content property:
using (DiscordWebhookClient client = new DiscordWebhookClient(WEBHOOK_URL))
{
ulong z = 42342340290226;
await client.ModifyMessageAsync(z, x =>
{
x.Content = "This is the updated message content";
});
}
If you want to update multiple properties at the same time, you can just add another line inside the lambda.