Not to be too negative, but rewinding that motor would be far more expensive than replacing it. And either way, you will need a control system capable of deciding which speed to run, and switching around the separate windings to change it. If this is, as I suspect, a spa pump as indicated in this link
, your existing spa controller will not have that capability. If you look down on that catalog to the next version, the 440 Series, it IS a 2 speed motor, just for the purpose you mentioned. But it still must be coordinated with a controller made for that, and mentions that in the catalog.
Find out what kind of motor it is. If it is a PSC (Permanent Split Capacitor) design, there are speed controllers that will work. If it is any type that has an external capacitor on it there is no way to reduce speed. You must either replace it with a 2 speed version as he showed, or reduce the speed externally as aolalde said. The catalog information does not indicate what type it is, and that box on top of it could be big enough to hold external capacitors. Open it up and take a look. If you don't know what a capacitpor looks like, look at the label for any designations that include the term "mf" (for microfarads). That would indicate it is a capacitor. if there is nothing but wires insidethere, it may be a PSC motor and you could put a VFD on it and use an external pot to change speed.
If all this sounds too complicated for you, live with it like it is.
"Venditori de oleum-vipera non vigere excordis populi"