Hey Graham,
Thanks for reaching out to us!
When purchasing, you have the right to use the License Key on a certain number of sites depending on the number of licenses that you have purchased. It looks like you’ve currently purchased a single license that allows for the license key to be activated on a single site.
To check where that key is currently used, check out https://theeventscalendar.com/license-keys/ and see the site associated with that key. If it isn’t the one that you would like to update, disable the active install at the link above and then you’ll be able to use your license key on the site that you would like updated.
Let me know if this helps.
Thanks!