First find the observation you wish to delete;

Scroll down to the list of observations, find and click the relevant observation.

Check the Unauthorised and Authorised tabs and click on the observation.

Saved only, will open directly into the observation, scroll to the bottom of the page and click Delete.

Authorised, will open the observation as a page, click Deauthorise in the top right corner. 

Once you are in the observation, scroll to the bottom and click Delete.