To upgrade simply first make backups of your styles by exporting them.
Next import XenBase 1.4.0 Beta 1 overwriting your current XenBase, then import the style you've purchased and overwrite it. All updates should trickle down to your child style.
This is possibly the last release we'll do for the 1.4 Beta series, from the past when XenForo enters a beta stage we see updates weekly sometimes which won't make sense time wise to keep pushing out updates.
Upon 1.4 stable we'll get the our version of XenBase out which we'll be adding some really neat stuff to it i think everyone will enjoy.
For any questions don't hesitate to ask, and always we really appreciate everyone's support.