How to upgrade iOS software

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. 

As i can see that you are running ios 10 , with iphone 6 which means you can update till ios 12 but if you have less storage then the update won't get downloaded .

Go in setting , swipe down and tap on "General" option then tap on "software update " and then wait till the iphone checking for update you will also see the circular moving cursor.

And if it still wont show try again , but this time connect your phone to charger and also to WIFI(with good network).

NOTE- you should be connected to the network ,otherwise the update won't show up

Also , check your storage atleast there should be 2gb free space in your iphone.

If you have an iPhone 6.....as your profile indicates....save yourself a lot of reading.

iOS 12.5.5 is the end of the road for the iPhone 6. The phone cannot be updated any further.