How do I change the welcome message?

The welcome message in the top left hand corner of the dash can be customised in the Site content area via Admin>System>Site content. Select the text you’d like to update and enter it into the ‘Search text’ box. 

Once you’ve found the relevant text, click through to make changes to it. Different messages can even be used for specific user groups, for example providing general users with useful admin contact information. Select the group you’d like the new text to show for before saving.

A full tutorial is available here.