Just updated my W3 to 8.1 through the MS store and a couple of notes:
1) Has to have KB 2871389 installed for the upgrade option to appear in the store. Then it is the first thing you see.
2) After selection it just sat there for almost an hour before the download started (Store may be queuing). Could see updater just sitting there in Task Manager so knew something was happening.
3) Once started it took about 1/2 hour to download (with a 45 Mb/s pipe) 2.7GB and install took another hour. Spends a lot of time just sitting there.
4) After load it logs in and then if you used a local login it really wants you to give them a MS account. To avoid click on "Create new account" and at the bottom of that screen will be a little option to "Use local account". Personally I do not care for the cloud.
5) Before install I had about 9 GB free of 32 GB. Afterwards I had nearly 11.
So took a very long time the first hour of which nothing seemed be happening unlike when I upgraded the W510 at midnight. May be because was in middle of day. Fortunately I could just leave it and work on other machines. On completion everything including touch and wireless were working. No need to reapply any Acer drivers.
In fact wireless speed no longer seemed impacted by an external 1920x1080 monitor & was getting 16 Mb/s down. Much better (4 bars rather than 2 originally) but still a lot slower than when wired (45 Mb/s). Think it updgraded the Targus dock DisplayLink drivers as well.