How to hard reset my iphone 6s plus without computer

If you mean you want to wipe it clean, and delete all data on it, the only way to do so without a computer is to have access to Settings->General->About->Reset->Erase all Content and Settings

If you cannot access the iPhone, the you will require a computer to reset it.

If you cannot access the iPhone, then you will need a computer running itunes to reset it. No way around that.

It can be any computer, Mac or PC with an up to date version of iTunes.

If it's in recovery mode, your only option is to restore iOS using iTunes.

If you can not do it on the iPhone, then you need a computer as stated.