Hi Ian

Thanks for posting on the community! Unfortunately this feature along with many other of the advanced diagramming tools are not available on the mobile app. If you would like to see this feature in the future would you mind adding your idea to our feature request form? Ideas submitted to this form are passed on to the product development team for their consideration. 

In the meantime if you edit your diagram on the desktop broswer you will have full use of all of Lucidchart's features.

Thanks for your request and I apologize for any inconvenience this causes you.

That's a shame - seems like the mobile app is quite a way behind the web browser version.