The easiest way is to buy a chain breaker made for #415 chain. The best ones are the RLV breakers. A HF breaker is going to break... No pun.
If you're a cheap sob, then you'll need a hammer, a 1/4" nut, a point punch or Philips drill insert, and a pin, screw, punch whatever smaller than the pin you're removing. and a small socket like a 1/4". Put the nut under the pin and hammer it till it's flat with the side plate. Should take one blow. Then use the point punch, maybe using vise grips to hold it, to punch the pin through the outer side plate approx 2mm. Then replace the washer underneath with the small socket, and punch the pin with a small punch or screw almost all the way out, till it is out of the inner link but still in the outer side plate. Pull the chain apart. If you go too far you won't be able to get it back in the outer side plate. Do the same on the other side, then align the new small link between the outer plates with the pin in one, hammer in the pin, then put a washer under the other side and hammer it so the pin sticks out the side plates the same. I got really good at this and have even done it for a friend out on the road with minimal tools such as a socket, nut, micro screwdriver, and a 10lb rock as a hammer.
You should not be grinding on a chain you intend to re-use. The abrasive particles that end up on the rest of your chain will work themselves into the pins and rollers leading to uneven (and rapid) chain wear.