I think the WiFi card in the Stream is soldered to motherboard. That is second hand knolledge from someone who has taken appart a Stream 13. Also I think while 5Ghz is a quieter band I think beyond adding a card if you could you would need to upgrade antennas and that would be more added complication. Otherwise I think you would find the 5Ghz signal range very poor indeed. From my experience unless you plan to be close to a access point for Wifi. 2.4 ghz will be far better for range. This is why many public and community wifi access points will probably not widely use 5Ghz. 2.4 Ghz still has the range advantage.
I was wondering if the wifi card on the Stream 11 could be upgraded to a wifi card a 5GHz wifi card because i am thinking of getting this laptop for school uses and the school's wifi is 5GHz wifi.
Also I need to know where i can buy it in store. I live in Campbelltown, near Sydney, NSW, Australia
Yes I replaced mine! The HP Stream 11 has a "Half Mini PCIe" slot. I would recommend getting ether a IEEE 802.11ac or the Qualcomm Atheros AR9462 IEEE 802.11n + Bluetooth 4.0 (Free Software friendly) card.