You could try contacting sales@avast.com they should be able to point you in the right direction. I’m sure I have seen a renewal link in the forums for share*it or shareit you could try a search.

http://www.avast.com/eng/renewals.html

Other notes:
  1. This renewal process is not available for customers who purchased avast! products from a Distributor/Reseller. Please contact directly your Distributor/Reseller to renew your license.
  2. This offer is for a full line of avast! products excluding the avast! BART CD. Maintenance of the virus database for avast! BART CD - one year is 20% of the original price (i.e. you save 80% of the price). To get your discount, please contact sales@avast.com