As you activate your licenses through various sites the activations will show up on your profile on the manage license page in your profile on this website.
License management has a little section in your profile, directly under your order history.
Here you can oversee, reset, recover and re-link your license keys.
Oversee and reset
The “Manage License” page is useful for remembering where you use your plugins, should you forget. But also to reset your license if you are unable to de-activate your license on your site.
If you are completely stuck and nothing seems to work please get in touch via the contact form and explain what goes wrong. Please include your license key so I can look up the specific license for troubleshooting.