You have three options: cut a stitch and unravel; pull a thread, cut and gently pull apart; or cut the fabric and unravel back to a clean row. You can open a whole row, or just part.
Cut a stitch (more controlled, recommended for small areas and opening just part of a row):
Pull a thread (less controlled, recommended for opening a whole row; might be difficult with stiff or weak fabrics; be prepared to unravel a couple of rows):
Cut fabric (less controlled and messy; be prepared to unravel a few rows):