This page of the Disco Elysium guide contains a detailed walkthrough of the side quest: Wash of the death smell.
In the Wash of the death smell quest in Disco Elysium, you have to use a bathtub in the Whirling-In-Rags hotel room so that Harrier can wash away the unpleasant smell of death.
This task can only be done once Harry gains some privacy. It is best to complete this task at the end of the day when Kim goes back to his room.
Before you can use the bathtub, you need to fix the faucet first to prevent the mirror from clouding up. Equip your character with chaincutters, then interact with the mirror and pass the difficult Interfacing skill test.
Once the faucet is fixed, you will be able to interact with the bathtub and allow Harrier to take a bath (Run yourself a bath). The quest will be completed once Harrier comes out of the bathtub.