"How can I reset the Screenshot numbering in Win-10 Tech Preview?"

This is the very same as it was in Windows 8. Just use your Windows Key + R to open the little window where you type in Regedit and then click OK. Navigate to H Key Current User\Software\Microsoft\Windows\Current Version\Explorer, as shown above, and look in the list on the right for ScreenshotIndex. Right-click on that word (highlighted in blue above) and choose Modify. When that little window opens, change the number to 01, and then you can close the window, because you're done. Your screenshot counter is now reset to begin from one.
Just don't forget to delete all the screenshots in your folder before or right after you do this, (before saving more) so that everything goes as planned. I do a lot of screenshots, so I use this frequently. I'm glad it hasn't been changed.