Yellowing leaves aren't an uncommon malady in peace lilies. The problem is that this one symptom can have many causes. Overwatering is one, and so is underwatering. Nutrient deficiency may also be a factor, or the leaves may simply be dying as part of the peace lily's natural life cycle.
Look back on what you've been doing the past few weeks. Depending on the situation, repot the plant, add water, or incorporate fertilizer. Ideally, make just one change at a time so you can narrow down the problem.

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