Wi-fi problems. Please. G700?

I have a Lenovo G700, with 802.11b/g/n Wi-Fi connectivity.
My rooms upstairs and I use an extension to get wifi though I'm quite close to the lan downstairs considering.

It gives the illusion of full bars but in reality, if I were to watch a youtube video the auto-resolution is 144p D:
What can I do to fix this? Is there a way to upgrade a computer part or get a new extension or do I have to live with it:l

Many thanks
C

ps. Is there also something I could change so my computer doesn't freeze when I try to screencast?

Read more: Wi-fi problems. Please. G700?