1. Is there have documentation for guiding how use the source code ?
Documentation and source code are included in download package. You will get all in one.
2. Dose it support backend or connect any APIs ?
No, It dose not. It just template application only. We do not connect any APIs. You have to add APIs functions by yourself for matching with your business.

3. After buying this application, can i modify color, components, remove or add more functions?
Yes, you absolutely can do that.There are tutorials in the documentation

4. Can I use this application and connect with my backend APIs?
Yes, you just add more functions for connect it with your backend APIs

5. Will I get the free upgrade on next coming version ?
Yes, you will get the free upgrade with more awesome features coming, please follow the change logs to see the detail update. You will get the notify email for new version update on stores.