How can i increase my weight? my present weight is ...

I know it sounds weird, but it's actually easier to lose weight in a healthy way than it is to gain weight in a healthy way.  I would recommend that you see a nutritionist just once to see what he/she recommends.  That way you'll know that you aren't causing yourself health problems by your weight gain.  Good luck.

I assume you tried eating a bit more than you're used to. If not you can try this but going to a dietitian is a good idea. This is basically what they do so going to get some advice seems like the sensible thing to do.