Look in the thousands place, its a 7. Now look in the hundreds place. if the hundreds place is 5 or up you would round up 7 to 8 making it 78, 000, if the hundreds place is under 5 then you would round down to 0 and it would be 77,000.
ex: 54,397, if you round this to the nearest thousands you'd get 54,000 because the hundreds place is less than 5.
ex: 54,937, if you round this to the nearest thousands you'd get 55,000 because the hundreds place is 5 or more.
I'm sorry if you don't understand anything, I'm not that good at explaining stuff (;