can anyone help me, how to choose plan for developer.?
because I want to use my project only fro 4 weeks only.
so if I buy the plan, it quite wasting for me.
Is there any platform for that?
how to contact technical support as shown.
You click on the Blynk For Business link at the top right of this page. Or just click here 
http://www.blynk.io/
Then click on the Contact Us link… at top right of that page. Then maybe you and Blynk can work out something?? Just a guess, not a guarantee 
Otherwise Developer - Free means you do the development yourself using the freely available Blynk that most everyone here is using… we (other Blynk users here in the forums) can help with minor issues, as able, but it is your project for you to develop.
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Free Plan is regular Blynk App you use. If you need to export your app - you would need to choose a Startup or Business plan
By the way, you can read about publishing here:
How much for startup?
it’s very expensive. I’m student. Can i have a Free App
@nguyentrangiabao05 You are here already 
just install the Blynk App from Google Play or Apple’s App Store and start reading the documentation and Help Files…