How can the hemoglobin level in the body be increased?

Depends. Depends on the reason that it is low. About the only thing that you can do on your own is dietary. If you have anemia because of low iron or vitamin deficiency, then improving your diet may help.
Dietary iron. Eating dark greens or black strap molasses can increase your iron, especially if taken with orange juice. Iron supplements also work, but can cause constipation. Finally, red meats are an option.