The answer to 'how to fix supply line' is to first turn off the water supply to the affected line, then inspect the line for any visible cracks, leaks, or damage. If the supply line is damaged, you will need to replace it. To do this, disconnect the old line, measure the length needed for the replacement, and install the new supply line, reconnecting it to the water source and the fixture.
Supply lines should be checked regularly as part of routine home maintenance, and replaced if any issues are found. If you are not comfortable working with plumbing, it is best to call a professional plumber to handle the repair.