The F12 shortcut was given to Live Preview in CC2017 and continues to be one of the inaccessible keyboard shortcuts in the preferences, so there's no way built into the program interface to change it back to the old Preview in Browser. That doesn't mean you can't change it though. If you have full admin access to your computer, you can make a tiny text change to one of DW's program files to permanently give F12 back to Preview in Browser.
To re-map the old Preview in Browser (Open in Browser) to F12 rather than Live Preview...
1. Close DW and go to...
Windows: C: > Program Files > Adobe > Adobe Dreamweaver CC 2017 > configuration > Menus > MM
Mac: Applications > Adobe Dreamweaver CC 2017 > configuration > Menus > MM
(I think, I don't have a Mac to test)
2. Open the file "PIB_Dynamic.js"
2a. As always, make a backup in case something goes sideways. Add _old at the end of the file name.
3. Search for the text "dw.browseDocument". There are 3 spots with that code, but the first two are just comments. The right one should be on or about line 433, and change the text "isPIB" to "true"
The finished line of code will look like...
dw.browseDocument(dw.getDocumentPath('document'),theBrowser, "-1"/*help argument*/, false /*use temp doc*/, true);
4. Save the file to your desktop
5. Drag the file into the original location from Step 1. You will be prompted for your Admin credentials in order to make the copy/replacement