The answer to 'how to fix drain' is to first try clearing the drain using a plunger or a drain snake. If the clog is more stubborn, you may need to disassemble the drain pipe to remove the blockage. This involves unscrewing the drain cover, removing the P-trap, and using a wire coat hanger or other tool to dislodge the clog. Be sure to wear gloves and eye protection when working on drains, as there may be standing water and debris.
If the clog persists or you're not comfortable working on the drain yourself, it's best to call a professional plumber. They have the tools and expertise to safely and effectively clear the drain without causing further damage to your plumbing system.