[authorization] How to generate access token using refresh token through google drive API?

I have completed steps of authorization and obtained access token and refresh token.

What should I do next to generate access token using refresh token that I have stored through google drive API?

I won't be able to use any sdk since I am working on Force.com so please suggest the way to implement it directly through the API.

If you are using web api then you should make a http POST call to URL : https://www.googleapis.com/oauth2/v4/token with following request body

client_id: <YOUR_CLIENT_ID>
client_secret: <YOUR_CLIENT_SECRET>
refresh_token: <REFRESH_TOKEN_FOR_THE_USER>
grant_type: refresh_token

refresh token never expires so you can use it any number of times. The response will be a JSON like this:

{
  "access_token": "your refreshed access token",
  "expires_in": 3599,
  "scope": "Set of scope which you have given",
  "token_type": "Bearer"
}