You need to enter your Account Settings and select Your subscription and account.
There, you can see your current limits and extend them by clicking on “+” in the “Buy more” column. Once you’ve added extra keywords or users, click on the Save new limits button.
Please remember that extra keywords stay in your account, so if you need those just for one campaign - come back here when it's over and change the limits to old ones. Prices stated next to limits are monthly prices.
You can always reach out to your Account Manager if you have any doubts about this!