As worn is that coin is, any cleaning you do to it will likely make it worse. It is what it is.
Here in my area, the nickels come up really red from the soil, so what I do is scrub with a soft toothbrush all the patina off the coin (dark stuff), revealing the red underneath (this is normally a no-no in terms of other coins, like copper ones). Then I use the oil that is already on my fingers naturally to very lightly highlight the high points in the design. It allows me to see the detail as long as I don't get any oil on the low spots.
I would leave tht one alone. Don't wanna do something drastic to your first Shield. It is what it is.
Best Wishes,
Buckles