Often the schematic for wiring is on back of furnace cover(s).
And, the wires themselves can be labelled. Low/Low Medium/High Medium/High are my choices I believe. The unused wires will be of same gauge and should be capped. If you increase existing speed to a higher speed, by swapping wires, you may find it's noisier at the registers.
To experts on DIYChatroom/HVAC: 1) Does increasing blower speed get rooms warmer quicker? More circulation? 2) Is it true that if motor windings burn out on one speed of a multi-speed fan motor, by switching to another speed you will have a motor with new life (assumed maintained in all other ways)?