No. Simply changing the gear without doing anything else won't slow a bicycle down. You can coast along without peddling and that will slow you down or you can use the brakes to slow the bike. Changing gears will affect the speed only if the current speed is slower than the highest speed you can reasonably attain with the selected gear - changing to a higher gear will help you speed up.
THey slow it down, if the rider so wishes.
Bicycle brakes allow the rider to slow down in a controlled manner and with out undue wear on the tires.
Upshifting or down? Upshifting, no, just like a car. Downshifting depends on where you are in the RPM range. So long as you are no where near redline, you should be able to downshift the transmission to the next lower gear or even two.
due to force of friction
That is called friction.
NO... you should use a lower gear if you want to slow down not higher.
Speed up
You will slow down. If you down-shift from 2nd to 1st, you will slow down somewhat. If you down-shift from 4th, 5th or 6th gear, you will slow down very dramatically. It would give you a similar feeling as if you had slammed on the brakes and locked up the wheels. Not recommended.
Birds slow down by changing the position of their wings to catch the air like a parachute.
When you want to slow down.
no, you cannot
applying brakes on a bicycle to slow down. that's friction because when you apply the brakes, the tires of the bicycle slide on the road causing friction on the road from your tires to slow you down faster