I bought Lightroom 6 with product code. I changed to Win 10 and it is not working.

Registration is one time only.  You do NOT need to register your serial number again.

You must activate the software to use it. Activation is limited to just 2 machines at one time.  On how many machines did you install LR 6?

When you can't deactivate a serial number
BE SURE TO ONLY ASK FOR ACTIVATION SUPPORT TO ADJUST YOUR ACTIVATION COUNT
You may need to PERSIST or TRY AGAIN to find a support agent with more experience

This is a public forum, you need the link to Adobe support, not other users
Be sure to remain signed in with your Adobe ID before accessing the link
Do be aware that Adobe's staffing is impacted by the virus, so wait times are long
-make sure that your browser does NOT block ads, scripts, or pop-ups
-you must also allow 'cookies' in your web browser for the link to work
This link will go directly to the chat option
Type "Agent" in the chat box to bypass the chatbot and be connected to a person
-Meaning type in the word Agent without the quote " " marks
https://helpx.adobe.com/contact.html?rghtup=autoOpen
- or https://helpx.adobe.com/contact/phone.html
.
Also: beware of answering anyone who sends you a private message
-click for more information about scammers https://tinyurl.com/10791730