The answer to 'how to fix drain' is to first identify the cause of the clog or drainage issue. This could be a buildup of hair, soap scum, food particles, or other debris. The first step is to try a plunger or drain snake to clear the blockage.
If the clog persists, you may need to remove the drain cover or disassemble the trap under the sink to clear the line. Be sure to wear gloves and eye protection, and work slowly and carefully to avoid damaging the pipes. If you are unable to clear the drain yourself, it's best to call a professional plumber who can diagnose and fix the issue properly.