It's pretty straight forward to swap springs. You can get the S&W shop manual at www.gunbooks.com , the best $20 you'll ever spend. Spring kits can be had at www.gunsprings.com , and a good set of screwdrivers (or all 3 items) at www.brownells.com
Once you know what you are doing the whole process takes less than 5 minutes.
On an defensive gun like the 640, you may want to start with just swapping the trigger return spring and leave the mainspring stock. A #13 return spring is a good place to start.
Tip: a #2 Phillips screwdriver works great to re-install the rebound slide/return spring.
