When major releases come out, they are initially set for self-initiated downloads by enthusiasts who want to update to new version. After a brief period of stabilization, the release level is marked to show an update is needed. So, in time, your 102 will show that an update is needed and that will proceed automatically or under your approval, whichever you have set. OR, if you want to update immediately, you can download the installation file and install over your current version. I do suggest doing a backup now, so that you are protected against any unanticipated problems. So far, the release is going smoothly. My suggestion is to wait. Several aspects are different and the screen will look different. I suggest you browse this URL for an overview: https://support.mozilla.org/en-US/kb/getting-started-thunderbird-main-window-supernova#w_2-unified-toolbar