code: public String createOAuth2AccessToken(DbxOAuth1AccessToken token)
          throws DbxException
      {
          if (token == null) throw new IllegalArgumentException("'token' can't be null");
          return DbxRequestUtil.doPostNoAuth(
              requestConfig,
              DbxClientV1.USER_AGENT_ID,
              appInfo.getHost().getApi(),
              "1/oauth2/token_from_oauth1",
              null,
              getHeaders(token),
              new DbxRequestUtil.ResponseHandler<String>() {
                  @Override
                  public String handle(HttpRequestor.Response response) throws DbxException {
                      if (response.getStatusCode() != 200) throw DbxRequestUtil.unexpectedStatus(response);
                      return DbxRequestUtil.readJsonFromResponse(ResponseReader, response);
                  }
              }
          );
      }
docs: Given an existing active OAuth 1 access token, make a Dropbox API call to get a new OAuth 2
  access token that represents the same user and app.