in uber-core/src/main/java/com/uber/sdk/core/auth/AccessTokenAuthenticator.java [113:119]
AccessToken refreshToken(AccessToken auth2Token) throws IOException {
AccessToken newToken = auth2Service.refresh(auth2Token.getRefreshToken(),
sessionConfiguration.getClientId())
.execute().body();
tokenStorage.setAccessToken(newToken);
return newToken;
}