How to create a instance of UserCredential if I already have the value of Access Token?

Assuming you already have the tokens you can do the following

string[] scopes = new string[] {
    PlusService.Scope.PlusLogin,
    PlusService.Scope.UserinfoEmail,
    PlusService.Scope.UserinfoProfile
};

var flow = new GoogleAuthorizationCodeFlow(new GoogleAuthorizationCodeFlow.Initializer
{
    ClientSecrets = new ClientSecrets
    {
        ClientId = "xxx.apps.googleusercontent.com",
        ClientSecret = "xxxx"
    },
    Scopes = scopes,
    DataStore = new FileDataStore("Store")
});

var token = new TokenResponse { 
    AccessToken = "[your_access_token_here]",
    RefreshToken = "[your_refresh_token_here]"
};

var credential = new UserCredential(flow, Environment.UserName, token); 

Then you can pass your credentials to the service's initializer

Reference Google API Client Libraries > .NET > OAuth 2.0

We can create GoogleCredential directly from the access token and use it instead of UserCredential as well. I just think it might be helpful for someone. Here is a code snippet of GmailService initialization:

GoogleCredential cred = GoogleCredential.FromAccessToken(accessToken);
GmailService service = new GmailService(new BaseClientService.Initializer {HttpClientInitializer = cred});