Pretty much any folder you prefer to use that isn’t inside Program Files (inside Renoise’s protected program folder, to be exact).
Maybe your Documents folder?
Maybe C:\Audio\XRNI\ ?
It really doesn’t matter — Just pick one that makes sense to you.
In Renoise’s Disk Browser you can navigate to your custom folder, then right-click one of the 1/2/3/4 preset buttons to save that location for easy recall later.
OK - done a little bit more clicking round, and found a small ‘head’ icon in far right panel by content loader window. This goes to ‘User Library’: (C:\Users\McLovin’\Documents\Renoise\User Library\Instruments).
The button under this goes straight back to Renoise bundled Instruments.