Welcome! Glad to have you with us.
I would assume that you want to "manually" check for updates?
This is very easy to do. Right click on the "Avast" ball in the tray. Click on Updates, select Database or Program, and then click OK.
Thats all there is to it!
Why, may I ask, do you wish to "check" for updates. Avast does this automatically for both Database and new Programs.
The default is to automatically download and install new database updates, and alert you to a new program update. I have my Avast set to automatically do both.
Hope this helps.