You're welcome
Yes you can go to Settings > System > License Key > Upgrade or if it's expired just enter the license key in the box after logging into the admin area.
You can only get the patches not upgrade to the newer versions if they have expired.
Say you have 3.5.x and your support and updates have expired, you can't upgrade to 4.x.x or 4.1.x or 4.2.x etc but you can use 3.5.1, 3.5.2, 3.5.3, 3.5.4, etc.
Unless you buy the Blesta Owned Lifetime license then you don't need to pay for support or updates at all.