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Wing 7 is on Steam: https://store.steampowered.com/app/1089000/Wing_Pro_7/ - presumably you would want that instead since buying Wing 6 won't get you an upgrade to Wing 8.
A Wing 7 license comes with a year of Support+Upgrades so you could upgrade to Wing 8. However, that's not on Steam yet so you would have to get Wing 8 from wingware.com. When you first run Wing 7 from Steam, it will fetch a license code automatically. After that, you can see the license in Wing's About box under the License tab and you can enter that same license code into Wing 8 downloaded from wingware.com.
I don't know yet when Wing 8 will be on Steam. There is usually a lag between final release of a new major version and its appearance on Steam.