You may only have one trade link active within the site. You can add and change the link from the "account settings" option on the top right of the screen.
In order to change the current link on site, the site will verify by email before the link can be reset. If you currently have items in your supply page, or are renting items, the link cannot be changed until the items are returned, or withdrawn.
Once you add a trade URL to a LootBear account, that account will be the only one able to use trade URLs from the same steam account (even if you generate new ones). This is in place in order to protect your Steam account.