If the oldest (bottom) leaves are yellowing, it is usually due to the plant getting dried out. If the plant wilts, you water it, the leaves "un-wilt", and then the yellowing occurs, it is from the drying out. Peace lilies really don't like to get dried out.
This doesn't usually happen to peace lilies, but if they are kept so wet that their roots start to rot, you can get yellowing leaves. If you pull the plant out of the pot and the roots are black, mushy, and/or smell bad, they are rotting. Healthy roots will be white or yellowish. If you have root rot, the plant will likely die. You can try repotting it in fresh soil and hope for the best.
If the leaves are turning yellow in small circles, that is, the leaf is green but there are yellow circles on the leaf, you may have an insect problem (most likely aphids) or a fungal or bacterial infection. Diseases will usually look like concentric rings. Diseases are hard to treat, it is usually easiest to just remove the affected leaves.
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