If the carpet is still under warranty, call your Installer. If not, it's best that a professional do this job.
If the carpet has enough 'play' in it, he'll streatch it and re-tape it. Request that he use a 6-inch seaming tape. The 'cheaper' 4-inch tape is not as good - It has a tendency to let seams peak. The 6-inch tape will bond, streatch, and shrink properly.
When the installer comes, be sure to empty out the room so he can do the job 'right'. If there's ANY furniture in the room, he may be back!
My best to ya and hope this helps.
PS: There may be repair 'kits' at the Home Centers. If you try this yourself, use a 6-inch tape and rent a kicker from a rental agency. BUT, before you go gun-ho, read the installation instructions / repair instructions at The Carpet & Rug Institute for what you need to do.