How do I cite an article from an encyclopedia?

Treat encyclopedia entries as chapters in an anthology. 

When you use an unsigned article (i.e. entry without authors) from an encyclopedia, you use the title of the entry as the reference both in the text and in the reference list. 

Example (adapted to LUSEM's Harvard Referencing Style Guide):

In text:
Change management theories are explained as... (Change Management, 2004)...

Reference list: 
Change Management. (2004). In G. R. Goethals, G. J. Sorenson & J. M. Burns (eds), Encyclopedia of Leadership, SAGE:Thousand Oaks, CA, pp. 155-157, Available online: https://dx.doi.org/10.4135/9781412952392.n40 [Accessed 8 Jun 2022]