I need better wifi in my house - options?

I have a pretty large (4500 sqft) two story house and wifi is served by the standard FIOS router that sits in my office.

The kids are about to kill me because the wifi is really spotty upstairs.

So - I know I need to extend the range of my wifi. What's the best recommendation without creating a bunch of segmented zones in the house?

I need better wifi in my house - options?