This depends on how inflexible it is.  You might have to start by simply trying to manipulate (move & stretch) your pinky with your other hand to get it moving.  Then try to move it on it's own, not necessarily fretting notes on the guitar.

But if you can already do that, then these tutorials are designed to help you develop strength & dexterity in your fretting hand & in particular your pinky.

Trill Drills is a series of exercises that focus on all fretting fingers.

https://www.guitartricks.com/tutorial/1829

Building Pinky Strength & Dexterity is a series of exercises focusing on your pinky.

https://www.guitartricks.com/tutorial/1292

Hope that helps!