The best way to rip out a seam is with a seam ripper. Keep in mind that seam rippers, like any blade, eventually dull and need to be replaced. While there are several ways to rip out a seam and several tools that can do the job, my favorite method is shown below:
Yikes! This seam needs to come out:
Use your seam ripper as shown to cut through every second or third stitch:
All done on this side.
Turn over and gently pull the uncut thread from the back side of the seam:
Pick up the little thread bits with a lint roller. Now, try re-sewing the seam.