Access token is expiring every day? How can i fix it?

Korefer87 Rich is correct. Dropbox issues short-lived access tokens, which expire after a few hours, and optional refresh tokens, which don't expire. Apps can get long-term access by requesting "offline" access, in which case the app receives a "refresh token" that can be used to retrieve new short-lived access tokens as needed, without further manual user intervention. Refresh tokens do not expire automatically and can be used repeatedly. You can find more information in the OAuth Guide and authorization documentation. There's a basic outline of processing this flow in this blog post which may serve as a useful example.
 
For examples of implementing this with the Dropbox JavaScript SDK, please refer to the examples included in the "examples" folder in the SDK. As long as you supply the necessary credentials, for instance, set the app key (a.k.a. client ID) and refresh token, like shown in this example for the JavaScript SDK, the SDK will actually handle refresh process for you automatically. (Note that the app secret is also required if the app does not use PKCE.) The SDK will automatically catch expired access token errors and call the API to get a new short-lived access token when needed.