When driving uphill in a vehicle with a manual transmission and you find your vehicle losing power and speed, you should:

Downshift

Explanation:

In a car there are a series of gears that transfer the power from the engine to the wheels of the cars. This is because the engine makes the most power at a certain rpm (revolution per minute). The gears offer a mechanical advantage to the vehicle i. e., the force is magnified by the application of some mechanical device.

When you are in the first gear the gear of the engine is larger than the gear of the transmission. So, with a slower rpm of the engine the rpm of the gear of the transmission is high providing a mechanical advantage. So, while going uphill you should always downshift.