The Replace Water Jug Task in Polus is a long task consisting of 2 stages. The location of the first stage is in Boiler Room. All you have to do is fill up a water jug by holding down the red button until all of the water is transferred from the water jug's right side to the left.
Stage 2 is in the Office Room, and now you will have to do the same thing as in the previous stage.