You should be prompted whenever there is a newer version of iTunes available to you. Perhaps it has been turned off.
If the top menu bar is not displayed in your iTunes, use CTRL+B to turn it on. That will display this:
Click on Help/Check for Updates. If you see a message similar to this one...
...then you already have the latest version. Otherwise, a prompt will appear to help you download the new version.
For the future, to ensure that you are prompted when a newer version is available, click on Edit/Preferences/Advanced>Check for new software updates automatically: