This is the general upgrading guide, please go the change log on the release note page to see if there is any breaking change or any other information that you need to be aware of.
First of all, things may go wrong for various reasons. Make sure to ALWAYS make a backup of both your site's files and the database so that you can return to a version that you know works. You will need the backed up files for merging later so this step is not optional.
It is easy to upgrade uBooking, just download and install the latest version on the project home page just like you did when you installed uBooking for the first time. When the installer runs it will replace all the uBooking files including the Views, Macro files, style sheets to the default ones, this means that you need to merge your changed views manually from your backed up files.
If you have edited the layout of the calendar you need to recreate the css file, go to Settings > Resource Layout and press the save button, this will automatically recreate the css file.