The key concept here is called Yield To Maturity (YTM). This is the yield that bond has when held until its redemption date. It is calculated from the coupon and the price the bond trades at today. (Which may not be face.) What happens is that as interest rates rise and fall, the price that a bond will buy or sell for adjusts so that the YTM matches the current YTM of new similar bonds.
Let's look at an example using some simple numbers. Suppose that we have two treasury bonds that have 5 years left on them. (They were issued at different times with different maturities.) One pays 4%, one pays 2%. Now suppose that the yield of newly issued 5 years treasuries is 5%. What will happen? The price of the two bonds will adjust down until the effective yield based on the price the bond trades for is 5%. The price of the 4% bond will have to fall by about 4.5% of face, and the price of the 2% will fall by about 13% of face.
It's always good remember that bond prices and interest rates are on a seesaw. As rates go up, price of existing bonds go down and vice versa.