BACK UP BACK UP BACK UP BACK UP BACK UP BACK UP BACK UP!!!!
We have built V2 so it will update V1 and still work correctly.
To update from Joomla Bookkeeping V1 on Joomla 2.5 to Joomla Bookkeeping V2 on Joomla 3.X
Update your Joomla 2.5 website to Joomla 3.6, then install Joomla Bookkeeping V2 over the top of V1. This should work perfectly ok for you and keep all of your data/transactions. But you MUST back up, as it is sometimes a real pain to update Joomla 2.5 to Joomla 3.X.
From Joomla V2 onwards (which is designed for Joomla 3.X+) onwards, it includes an auto update option, which allows you to update from your extensions update manager area in your Administrator area.
All the best
Our time is very tight, we work on this for free, have our own businesses and families, so if you are having trouble, please try and find if it is a simple solution using Google or the Joomla.org forums first, then drop us a message and we will do our best to reply as quick as we can.