You have a duplicate track number. It's a bit more involved than to normally drag and drop as suggested by ustoexx. With the album open, on select TRACK > SET TRACK INDEX. This should open the SET TRACK INDEX form.
You can try and use the up down arrows on each track to get the right track no, but my experience when track numbers get duplicated, has been that it is easier to click AUTO (lower right), click OK. Then drag/drop adjust then track accordingly.