The answer to 'how to fix supply line' is to first turn off the water supply to the affected line. This is typically done by locating the shut-off valve for that specific line and turning it clockwise to the closed position. Once the water is turned off, you can then unscrew the supply line from the fixture or appliance and inspect it for any cracks, leaks, or other damage. If the supply line is damaged, you'll need to replace it with a new one that is the same size and type. Be sure to use plumber's tape on the threaded connections to create a tight seal. Finally, turn the water back on slowly and check for any leaks before reassembling the fixture or appliance.
It's important to take safety precautions when working with plumbing, such as wearing gloves and eye protection. If you're not comfortable working on the supply line yourself, it's best to call a licensed plumber to handle the repair.