it depends on the size of your piano, the quality of your mics, the size and layout of your room, ambient noise, tempurature / humidity.
I would suggest (if your piano is an upright) to start by placing the mic close to the piano with the lid slighty propped up.
Play something and record it.
Listen to it, and if it sounds too muffled, move the mic again...play the exact piece as close to the last way you played it, record it.
It takes time to find that sweet spot. Make sure you play without any variations in volume, tempo, emphasis, etc. Because this can throw off your recording and could trick your ears.
I cannot tell you exactly where to put your mic because the given variables above are unknown to me.