Hi Marco, just download the zip archive from the website, unzip it and run openLCA. Your databases will show up in the new version of openLCA (unless you manually changed the folder in which openLCA saves databases).
Anyway, we always recommend you to make a backup of your most important databases once in a while (right-click -> backup database !!! ... just save them on GoogleDrive or in your Dropbox, external hard drive etc. ...). To be a hundred percent sure not to mess up your database during this update, maybe do so before starting openLCA 1.8 for the first time.
I hope this helps.