Sorry for my poor english, please don't kick me in the face!

First, you need to download a Hex Editor (HxD or Free Hex Editor Neo, you can easily find it by yourselfs)

Second, you need to start NG+ on any other difficulty (Preferably on normal or hard, to make it easier to find values ​​in the editor), after cutscenes go to first save point and save a game.

Run Hex Editor and open your savefile, you can find it here: steam\userdata\<user-id>\638970\remote.

You will see a bunch of incomprehensible numbers, on the left side you need 00000440.

If save done on hard difficulty, you'll see 00 00 02 02 00 00... values.
If its normal, you'll see 00 00 01 01 00 00...

00 00 - easy (that's why it's advisable to start NG+ on higher difficulties)
01 01 - normal
02 02 - hard
03 03 - legend

So you need to change this values to 03 03, then save a file.

Load the game and enjoy it!

*IMPORTANT*

With this method, you can still get the achievement for completing a game on Legend difficulty!