How to change Avast Internet Security - 3 PC, 1 year, to 1 PC, 1 year.

Hi
I want to change my current Avast Internet Security - 3 PC, 1 year, Renewal type: Automatic,
to only 1 PC, 1 year, Renewal type: Manual

Hopefully at the same time as the expiration date: 01/29/2017

There seems to be a bit difficult to get good instructions from the Web Help and other support so that this can easily be done.
Anyone know how to do?

If I buy a new Avast SecureLine - 1 PC, 1 year, Renewal Type: Manual and install it, how do AVAST know when the new subscription to take effect?

A little bit confusing for me
//Peter
SWEDEN

https://support.avast.com/support/tickets/new

Thanks!
//Peter

A subscription starts at the day of ordering (if the payment is received).