NEC 750C User Manual page 24

Slide the battery away from the arrow on the battery
5.
and remove the battery.
Put the new Lithium-Ion battery pack in the bay. Slide
6.
the battery in the direction of the arrow.
Firmly press the battery until it is securely in the bay.
7.
Replace the main battery bay cover and slide the battery
8.
cover lock to the locked position.
Getting Familiar with Your MobilePro 750C 2-11
Opening the battery bay cover
Inserting the battery