I want to update OS in my Iphone 6

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

If the above Apple article doesn’t help, please post back with relevant information. 

The iPhone 6 can be updated to iOS 12.5.5.

You can update to iOS 12.5.5.

That is the last one for a 6.

msalatheef wrote:

now I am unable to to down load BOB world App. I need to update OS in my iPhone 6

What is the existing version of iOS on your phone?

for updating at least IOS13.0