You need to update the anti-virus program every time an update comes out which for most programs is once a year and they label with year as edition. The program will normally give you a popup telling you that you need to update the program. If it is a bought program you will need to pay again for it annually to get the latest edition.
The free programs will also need updated and some like Avast want you to register. Avast just asks for an email address and I have never received email from them.
If you go to Avast and it needs updated it will give you a screen and give you two choices. The first is the free option and the second is a bought option that includes more than anti-virus by including a firewall, etc. and calling it Internet Security. If you do the free, they will take you thru a few screens trying to get you to buy.
In addition you need to update the signatures or definitions every week or two. This allows the program to catch the latest viruses and malware. You can go in the program and choose updates or you will get a popup on the taskbar you can click for updates or some programs will automatically update especially the commercial copies.
Keep the program and definitions and signatures up to date so you are protected.