Used the wrong color wood stain on a Cypress post
Typically, I find sanding is the most sure way to remove a staining mistake. To use a solvent or thinner will not remove enough of the wrong stain to make the difference you are looking for.
When sanding by electric sander, it will leave marks that will not be apparent until, you stain. Those marks will also make the stain look slightly different than the other posts, even when using the exact same stain. You should finish the sanding by hand and sand up and down the post with the grain, not across the posts the short way, across the grain. This will even out the swirl marks left by most electric sanders.
You would need to start with a relatively coarse grit, like 80G to start, to remove the bulk of the stain. Switch to a finer grit like 120G to remove the swirl marks from the 80G. Then move onto the hand sanding with 120G and maybe finish with 180G.
Since the other posts are typically not sanded before staining, they are never touched by sandpaper. This surface usually stains differently than a sanded surface, so the need for a fine sanding with the grain is important.