Can someone help me on how to charge this Battery?
Quick answer: connect a constant current supply, with suitable voltage limit (4.25Vx5=21.25V), and leave it until pack voltage is high again. Clearly this will take forever (4+ hours) to charge, and will invalidate any warranty with the pack etc.

Longer answer: while there are many different chargers, there are two ways to charge li-ion cells, either using constant current only, or constant current followed by constant voltage. The quick answer method uses the constant current method where it pushes current into the cells until they are charged. This is the method used in my of the basic battery powered equipment out there, such as vacuum cleaners and some basic drivers. The more complicated method, which is used in the more power sensitive devices, pushes a fixed current into the cells until they reach somewhere around 80-90%, and then a fixed voltage and let the physics of the cells control the current.

In your application, I would suggest that your best bet is to use a desk top power supply, current limit it to something around 0.5Amps, and leave it to charge the pack. This isn't exactly a clean idea, but it works.