Brown leaves on Peace lily
Browning leaf edges is a common problem with peace lilies. It can be caused by fertilizer salts or the salts in household water that passes through a water softener. Or by over- or under- watering which has damaged roots, leading to root rot.
Or by a root rot brought on by overcrowded plants.
Slide the rootball (roots and potting soil) out of the pot and take a look at the roots. If they are black, slimy, mushy and/or smell bad, that indicates root rot. You can try removing rotted roots and repot in fresh soil and one size bigger pots--of simply buy new plants.