Spring-roller window shades, or pull-down shades, have a metal tube and a piece of fabric that wraps around the tube to roll up or down. A coiled spring and ratchet work inside the tube to adjust the shade; an adjustable pin at one end and a stationary pin at the other allow the tube to turn. Over time, the spring inside a roller shade can wear out. Sometimes the shade spring just needs tension adjusting, but it can also break and need replacing.
Take the roller shade out of the mounting brackets in the window frame.
Grip the adjustable pin with pliers and turn clockwise to loosen the latch, or pawl, inside so you can remove the spring.
Pull out the pawl and spring.
Insert the new spring and replace the pawl.
Turn the pin counterclockwise with the pliers.
Return the roller shade to the mounting brackets.