When a new version of Flume is available, you will also receive a desktop notification at launch. It is always recommend to update Flume to the very latest version to take advantage of fixes, improvements, and new features.
You can read the entire Flume version history in the help documentation.
If you've already downloaded and are using Flume, you can check for new updates manually by selecting the Flume > Check for Updates… menu item, or simply relaunching Flume. If an update is available, you will be prompted to download and install it. If you are unable to use the in-app update process (eg. due to a launch crash issue), you can update Flume manually by downloading it from flume.app, and moving it to the same folder / location as your existing copy.
Whether you update in-app or manually, your preferences, accounts, and other data is preserved. In some instances due to security upgrades and changes, it may be required to log in to your accounts again after an update.