To change the current Presence Status click on your photo in the top right. You will directly have the option to select any build-in Presence Status or choose a predefined Presence Status. To setup a custom Presence Status click on + Advanced.
Setup an Advanced Presence Status
Any Advanced Presence Status has a build-in Presence Status and optionally a message and/or due date. The due date can be activated by selecting the checkbox and choosing the date and time. When the setup due date has passed the account will automatically be switched to Available again. The message will be shared with everyone (including external people that use your Kite link) depending on your ACL settings.