I don't really know what you mean by license is invalid if you already have a teamviewer 10 account with a license. But here are some links that may help you!
Here is where you can buy a new licence for teamviewer https://www.teamviewer.com/pricing/
Here is where you can download teamviewer 10 https://www.teamviewer.com/download/previous-versions/
I hope this helps,
Thank you for using teamviewer!
We was purchased TV 10 corporate and working well. Few weeks ago we have new devices and installed TV 10 on those PCs. I tried to follow this step "https://community.teamviewer.com/t5/Knowledge-Base/My-license-key-doesn-t-work-what-can-I-do/ta-p/47..." but the license keep saying not valid.
Right now we have upgraded our licence to tv 12 and resolved.