@rahulsharma1902, Dropbox API doesn't issue long lived access token anymore. All access token issuing at present are short lived and remain valid for 4 hours or so. For long term access, you need refresh token that doesn't expire automatic and can be used repeatedly whenever needed to "refresh" (regenerate actually) access token.
Don't mess application authentication and account (user) authentication (as you're doing)! They are different things and usually one of them cannot replace the other (as you're trying). Application authentication (app key and app secret) authenticates the application itself (as the name suggests), not some account (including the one associated with the application). Such authentication can be used only for activity unrelated to particular account content, but only to content public accessible already (like some shared links related operations). To perform account access, you need corresponding account related authentication that represent account owner granting for your application to manage the content (including your own) - access and refresh tokens.
You can let the user (again, including to yourself) to grant access permissions to your application to access owned account using OAuth flow (take a look here for a simplest example). In such a way your application ask the user for permission to access the content. Here you can Note offline access that will result in adding refresh token. If you're using some supported SDK initializing the client object properly using refresh token is enough (you don't need to perform refresh yourself), otherwise the last step in the post I linked to be comes your responsibility.
Hope this helps.
