Lightening a dark wood stain involves either removing some of the existing stain or applying a lighter stain over it. Here are a few methods you can try:

1. Sand the Surface:
2. Use fine-grit sandpaper to gently sand the stained wood surface. This will remove a thin layer of the stained wood.
3. Be careful not to sand too aggressively, as you might damage the wood or create uneven patches.
4. Chemical Strippers:
5. Apply a wood stain remover or paint stripper following the manufacturer's instructions.
6. After allowing it to sit for the recommended time, use a scraper or steel wool to remove the loosened stain.
7. Wipe the surface with a clean, damp cloth to remove any residue.
8. Bleaching Agents:
9. Oxalic acid or chlorine bleach can be used to lighten wood. Mix the bleach with water following the instructions on the product.
10. Apply the solution to the wood using a brush, sponge, or rag.
11. Allow it to sit for the recommended time and then neutralize the bleach by wiping the wood with a damp cloth.
12. Vinegar and Water:
13. Mix equal parts white vinegar and water.
14. Apply the solution to the stained wood and let it sit for a while.
15. Wipe off the solution with a clean, damp cloth.
16. Apply a Lighter Stain:
17. If the above methods don't lighten the wood enough, you can apply a lighter wood stain over the existing one.
18. Sand the surface lightly to promote better adhesion.
19. Apply the new stain according to the manufacturer's instructions.

Always test any method in an inconspicuous area first to ensure the desired results and to avoid damaging the wood. Additionally, wear appropriate protective gear, such as gloves and eye protection, when working with chemical strippers or bleach.