Yield at maturity is fixed so if interest rate goes down market price of the bond increase or vice-versa.

Lets see this with example -

There is a bond of $100 for two years at interest rate of 10% means at the end of two years bond holder has the right to receive $121.

If interest rate remains constant the bond will trade at $110 after one year. Now say if interest rate goes down to 5% (from 10%) then bond will trade close to $115 because market is expecting 5% return and vice-versa.