Aside from the possibility of maybe a good whack across the backside may fix it, I'd suggest taking it to a service tech. Something may have slipped under the keyboard and is bridging the keys.
First off though, maybe try picking it up, turning it upside down and giving it a good shake and listening for loose parts inside should be the first thing you do.
It may just need a good cleaning. Shaking the crud to a new position may be enough to fix the problem or else spend the money and have a tech open it up and do a proper cleaning.
Unless you're a DIY guy. Then you may want to tackle opening it up and cleaning it yourself.
In that case, do a youtube search on "Yamaha Keyboard Repair" and you'll get some videos to coach you on how to open up the case and disassemble the keyboard.
That would be my approach anyway...
Gary