No matter how stubbornly filament is tangled, one can always undo a knot without unwinding the whole spool. In fact, unwinding typically worsens the problem as the knot simply becomes more and more tightened through pulling the filament off the spool.
To get the knot out of the spool, you need to lift a few turns, including the knot, from the spool. If you lift too few turns and you can see three paths coming from the spool, then you haven’t caught the knot. More windings have to be removed until only a single path leaves the spool. Afterward, simply rewind the filament under tension again and undo the knot when you reach it.
An alternative method is to step-by-step unwind the filament while maintaining pressure on the point where the strand leaves the other spool. In other words, one hand will be both holding the spool and pressing — with one finger — the loose strand against the underlying coils. Everything “behind” this finger should be tight. Then, with the other hand, take hold of the loose strand, maintaining tension, and slide the pressing finger back along the strand. Continue doing this until the knot is directly behind your finger. Then, instead of sliding the finger back (this would only push the knot back), grab the strand from behind the knot, and pull the loose strand all the way through! Lastly, rewind the unwound filament under tension.