The legend difficulty will auto unlock when you are finishing the game beforehand. However. If you want to unlock legend difficulty. Here is the step by step guide.
*NOTE: Modify the game file before you make the backups.
To do this, all you need to do is a simple hex edit on a file called system.sys. This will be located in:
Steam\userdata\########\834530\remote NOTE: the #### is placed as the folder number appears to be different per person. For me, there were only two folders in userdata and it was the top one. Just check either to find the folder 834530
Once you’ve located the file, make a back up just in case. From there, open an hex editor. If you do not have one downloaded, you can use an online one. A quick google search will help you find one in about a second. I found one called Onlinehexeditor and that did the job.
Upload the file to the site or open it with your programme, from there you want to look at the top row and under 04, change the number from 00 to 01.
If you are using the same website as me, do this by changing 0x0 to 0x1 in the hex8 tab on the side and click enter. Then save the file and replace it with the original system.sys in the folder.
It should look like this once completed:
Once finished, launch the game. Your menu should now look like this and from there you can play the game on Legend difficulty and cry on the Car Chase scene. Enjoy!
You can also find this guide on Steam