Do not buy an extender, waste of money and poor performance. New antennae and adapters will not help.
Your best possible solution is to run an Ethernet cable from downstairs to upstairs, then adding an access point is optional.
If a cable is impossible, then get a pair of really good powerline adapters (the new 600Mbps models -- they don't really do 600MBps, but about 100Mbps), like the Zyxel PLA5205kit.
The only other good option is to buy a pair of ASUS RT-AC66U or RT-AC68U routers and use one in router mode and the second in media bridge mode, their 802.11ac 5GHz 80MHz wide connection will give you a good signal -- but that is the high cost option.