To change the default store in CStore Essentials use the following steps:
Log into your CStore Essentials account
Click on the User Settings option located at the top right, click on Manage my store, then click on Users (at the top right hand side corner of the screen bellow the User Settings icon) and click on the Linked Users tab (on the left hand side of the screen).
Find the username and click on the "STAR" icon to make it the new default store.