How to use 'refresh_token' in IdentityServer 4?

I'm using .net core with IdentityServer 4. I have a Web api, and an MVC app which accesses secure endpoints on the api. It's very similar in setup to the IdentityServer quickstart:

https://github.com/IdentityServer/IdentityServer4.Samples/tree/release/Quickstarts/6_AspNetIdentity

I'm finding that my access_tokens are expiring, and I'd like to understand how to renegotiate refresh_tokens.

Take the following code for example (taken from the quickstart here):

public async Task<IActionResult> CallApiUsingUserAccessToken()
    {
        var accessToken = await HttpContext.Authentication.GetTokenAsync("access_token");

        var client = new HttpClient();
        client.SetBearerToken(accessToken);
        var content = await client.GetStringAsync("http://localhost:5001/identity");

        ViewBag.Json = JArray.Parse(content).ToString();
        return View("json");
    }

If the access_token has expired, it will fail with 401 response. Is there a built-in mechanism for re-negotiating the access_token using the refresh_token?