Based on the information you provided you seem to be using v3.x of auth0.net given that the GetAccessTokenAsync method was removed in v4.x of the library.
In version 3.x you could use a refresh token through the method GetDelegationTokenAsync() while passing a RefreshTokenDelegationRequest object.
If you’re implementing this in a new application you may want to consider starting with v4.x of the library instead. You can see the differences between versions and the scenarios where you should use one version over the other at: Migration Guide