How to update my iPhone in order to download the Facebook app in it?

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Stellasaypshn2005 wrote:

Yes how can i

As you have provided zero information as to what device you have, what version of iOS you have, any error messages, etc, it is difficult to offer further advice. But at least you were able to solve your issue.