Update 3.8.1

Hi I have a website http://tiramisuboutique.com.sg (version 3.1.2) , used to maintain by outsource designer. However we has not been using it from 2 years.

Presently we would like to update to 3.8.1but we cant find ‘update’ keys on our dashboard. How can we update? And how to change our theme?

Thank you.