To set yourself as unavailable, navigate to Account Settings>Profile.
This is how the Profile tab looks like under Account Settings in the web browser
You can also set availability on the mobile app by navigating to
Settings>Set Availability
You can mark yourself unavailable for a set number of hours or until a specific date and time. Once you've made your changes, click the "Update availability" button to save.
If you anticipate setting yourself as unavailable for a long period of time, please contact our customer support team to let them know.
See also: Adjusting Your Notification Settings