Taking the valvecap off for a day won’t make it leak out pressure. The valvecap is there to protect the valve stem.
To take out pressure simply press on the needle in the middle of the valve stem. Ideally you would use a tire pressure gauge to lower the pressure, however, your finger or a pen will be fine.
Post up your weight so people can give you a more accurate estimate of tire pressure to use. It will probably be somewhere in between 20-30. If you’re riding off road where you don’t want to bounce off of roots and different things on the ground make sure the pressure is low. Always make sure you have enough pressure in the tire to you won’t tap the wheel when you do a drop, jump, whatever. If you’re gapping and the tire folds over, more air.
Try lowering it to 30 psi and see how that feels, it’s always easier to take more air out than it is to put it back in. Go to 30, then lower to where you like it.