Install / upgrade instruction can be found here: eventbookingdoc.joomservices.com/getting-started/installation
As for your questions regarding losing data, all registrant / event data is stored in your Joomla database. No data is lost on upgrade.
HOWEVER, if you have custom code modifications, you will have to reapply the changes as all Event Booking *FILES* are overwritten on upgrade. CSS code with the /media/com_eventbooking/assets/css/custom.css file is NOT overwritten, along with Translations, Messages and Emails, and other settings available through the Dashboard.
If you have modified code applied as template overrides, you might have to manually delete the override files (override files contain old code from old versions of EB, and some new features won't work if the old override is still in place), or if you still want the override code to function, reapply the modified code to new overrides based off the newest version of EB.
James Riley .: EventBooking user since 2014 ::: JoomDonation user since 2016 :.
.: grfx & web design / IT / AV @ St. Therese Institute of Faith and Mission, Bruno, SK, Canada :.
Sorry, one more bug causes by change in Joomla core in version 3.5.0. Permission setting for Registrants Management and View Registrants List could not be saved in 3.5.0 and 3.5.1. I made a fix but Joomla team decided only include that fix in Joomla 3.5.2 (Joomla 3.5.1 will be releasing soon to address some more urgent bugs). For more details, please see github.com/joomla/joomla-cms/pull/9648
Because the fix is delayed, I changed Events Booking code to work with this bug. So if you have this issue, just re-download Events Booking and upgrade it to your site