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Refreshing an Access Token
If the application is using the Web server or user-agent OAuth authentication flows, a refresh token may be provided during authorization that can be used to get a new access token. The refresh token the client application already received.

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POST /token HTTP/1.1 Host: server.example.com Content-Type: application/x-www-form-urlencoded grant_type=refresh_token&refresh_token=[REFRESH_TOKEN] &client_id=[APP_KEY]&client_secret=[APP_SECRET]