1. If you don't know how to root a phone without using an app, you shouldn't - you could end up with a virus or a dead phone.
2. Rooting depends on two things - the phone you're rooting, and the version of Android it's running - so just telling us it's an S6 doesn't give us enough information to answer your question.
The best way to root it (especially without our knowing the version of Android)? Install TWRP. Back everything up. Open the backup (in a PC is easier). Find the system folder, then find the xbin folder in that. Find a copy of su that will run in an S6 and add it to the xbin folder. Save the backup (and copy it back to the phone). Restore from the backup. Install SuperSU or SuperUser from the Play store.
You're rooted. (Now you can't update the phone. You can use TWRP to restore the unrooted backup, update, back up the update and do it all over again to root the backup.)