How do I continue use ?

[gammypam]

 I purchased by used computer from a legitimate computer store. Avast 4.8 home edition antivirus was installed on the computer. I am now getting pop-ups that say the license has expired. I like the program and want to continue using it. Please instruct me on the steps that  I need to do to continue using the program. I am a senior citizen beginner computer user. Thank you in advance for answering my concerns. 
GammyPam

[DavidR]

- Complete the avast! 4 Home Registration form and you will receive an email from register @ avast.com (without the spaces) with your registration key.

Also see, Insert Registration Key - Video