Once the drywall tape has actually released from the joint, about the only thing that will really fix it is to cut out the defective length and mud in some new tape. This will take 3 or 4 thin applications of drywall mud (at wider and wider swaths) to get it set. Then you need to lightly sand, prime and paint.
If you do not feel up to this task, in most major metropolitan areas there are actually companies who specialize in just drywall repair. Drywall Doctor is one in my area, but I'm sure there are several others. If you don't already have the tools, the tape and the compound on hand, it may actually be about the same cost to pay one of these guys to make the repair as it would be to make the investment in the equipment. AND, you can stay off a ladder and avoid some sore shoulders! That's always a bonus.