A recent blog notes that the 8.1 update will be free and available through the Windows Store.
Look out around the 26th June for a public preview:
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application
continualupdates
customer experience
dell xps 12
developer conference
device selection
free update
game apps
lenovo yoga
microsoft store
mobile computing
new features
oem partnerships
performance update
public preview
sony vaio tap
tablet experience
touch
windows 8
windows 8.1
