How to deal with the license of a forked repo ?

Do as it says. If it gives you the permission to change it, do so. If it says it says, it stays.

@Kimmax and how to license the code that I've added to forked repo ?
Just add a copyright thing in start of file that I've written ?

@nihalmurmu depends on the original license. Maybe relicensing isn't even possible

Here is the thing, if it's a public repository it is open source by default so it should probably be using one of these: https://choosealicense.com/licenses... , figure out which one it is and read it, all your questions should be covered in the license, if it does not cover something then just ask the author of the repo