Refresh token and Access token in facebook API

For offline access, you need to exchange your short-lived access token for a new access token, before it expires. Facebook has a single type of access token (no refresh tokens). A about-to-expire access token should fetch you a new access token.

To manually extend the tokens using a Graph API endpoint ::

GET /oauth/access_token?  
    grant_type=fb_exchange_token&           
    client_id={app-id}&
    client_secret={app-secret}&
    fb_exchange_token={short-lived-token}

Quoting FB's documentation from here ::

Apps are unable to exchange an expired short-lived token for a long-lived token. The flow above only works with short-lived tokens that are still valid. Once they expire, your app must send the user through the login flow again.

Do read the Expiration and Extending Tokens portion of the documentation link that I have mentioned for further clarification.