Question

how to un block people on this

Answer 1

If you're trying to unblock him/her from inbox, you won't be able to, because his/her messages are deleted temporarily from your inbox.
Let's say you want to unblock prakhargupta3301, then you search prackhargupta3301 and see my profile, open it and click/tap on unblock person right beside the profile picture. There is no other way.
If you still need help, just ask in the comments.

Answer 2

You can easily unblock anyone.but you should have the user ID of the person u want to unblock. I can get the user from anyone who may have him/her in his/her friends list.
You just need to open Brainly.in site in any browser and search for the id u want to unblock. When the profile of the person opens, click write message button and there you will get the option to unblock . See I have uploaded the screenshot of your profile and in the right-hand side you get the button 'write message'.