Windows 10 has been changed to be a recommended update for users of Windows 7 and 8.1. This means that, under default settings for updates, it is going to automatically download and prompt you to install.
If you make the decision to update to Windows 10 on your point of sale computers, be aware that there are risks in doing so of breaking configurations. You'll want to make certain that you have a current backup of the database not on the computer. You'll also want to do this on a day where support is available, in the event that minor cleanup is necessary.
The most frequent problems reported involve the printer drivers breaking. You may need to reinstall printer drivers to fix this, even if the printer drivers are compatible.
If you do NOT want to update to Windows 10, then instructions on how to block the updates appear at