Want to update Samsung Galaxy On7 to Nougat I want to update my Samsung Galaxy On7 to android 7.0 Nougat.

The OTA updates haven't been rolled out.

I want to use all legal software that won't void my warranty or get me into Police trouble.

I found this link: http://www.android.gs/update-samsung-galaxy-on7/

I have Android sdk on my computer (all the build tools, and Android versions 5.0.0 to 6.0, and will download nougat also if necessary.

Can anyone please see the link and tell me whether it is safe and legal?

If not, then please suggest any other legal method.

I want to use no custom ROM.
Yes, that links seems safe to me. It tells you to download the official firmware for your device from the official Samsung page or SamMobile, which is a trustworthy website. The problem is, you won't find the Nougat firmware there either, because it's not released yet. The only thing you can do is wait until Samsung officially publishes Android 7 for your device. 

By the way, flashing any kind of Android firmware is never illegal, since Android is open source (only exception is when paid content is included for free). It can only void your warranty.