This is a simple NO UMD FiX by Reborn:
1) Extract Ys_6_The_Ark_of_Napishtim_USA_PSP-DMU to a folder on your PC using iSOBuster or similar utility.
2) From your original PSP_GAME folder move ICON0.PNG, PARAM.SFO, PIC0.PNG, and PIC1.PNG to the YSARKNAP folder from this pack.
3) Next, from your original PSP_GAME\USRDIR move the data and kmodule folders (including all of their contents) over to the YSARKNAP\USRDIR folder from this pack.
4) Now, from your original PSP_GAME\USRDIR\module folder move ONLY the libsha224.prx file to the YSARKNAP\USRDIR\module folder from this pack (there will be other files already in the folder).
5) Now follow the instructions as normal from the included NFO file:
Copy all folders contained in this release to the root of your memory stick.
Now boot using RunUMD.
NOTE: Although this is labeled a "NO UMD" release, that just means you don't need the actual Ys 6 UMD in the drive to play the game, however, you will need a UMD in the drive to run it (any UMD game should work).