A couple of reasons it might be staying in 1st gear are a bad tune or the shift point adjustment needs to be lowered. We need some more info. What's the engine temp, gearing(spur/pinion), tires, where and how your running it, ect...
On most transmissions, there is a pawl that comes out and engages the second gear when the pawl (usually it rides on the same shaft as the first gear, which rides on a one-way bearing) reaches a certain RPM. This is what causes it to shift. You adjust the shift point by turning the small grub screw in the pawl drum that presses on a spring, which in turn applies pressure to the pawl to keep it from engaging.
If you don't have enough RPMs then the transmission won't shift. You could try lowering the shift point by turning the screw counterclockwise. Chances are that you may have a loose slipper clutch or a badly tuned engine. Both will cause the tranny not to shift. If the slipper is loose (stick a small allen wrench in the hole on the shaft with the spur gear and spin the gear. It should be hard but not impossible to turn the gear), try turning the nut in the middle of the spur clockwise 1/8 turn.
If you give us a video of the truck we can see if the tune is bad.