Not much. Some routers can be configured as repeaters, but it's kind of a case by case thing, so you'd have to look at the other router to see if that would even be an option.
Changing the channel only helps if there is some kind of interference, like from a neighbor's router, and generally speaking 11n has a wider coverage area than 11g. So, really your only options would be to move either the router or the upstairs computer (or both), or run an ethernet cable from the router to the upstairs computer.
My router/gateway from AT&T is in my basement. so upstairs its a really weak signal. I just changed the channel 11, and the mode to 802.11g. What else can i do? there is no antenna and i have an extra router up stairs if something can be done with that.

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