Copy/paste the achievement/description here and edit it within the post. A mod will grab the info and just paste it over the old. Would take me like 30 seconds to fix it and most of that time is scrolling down in the guide looking for the achievement to fix.
You can do it any way you want, but I personally prefer you just make one post with the achievement. The tile doesn't matter if you don't want to include that.
In my example below, all I do is list the game and why it needs edited. I then pasted my solution down. I am only using red on what I added just to point out the example.Again, I just pasted the description I had in my guide. Then I added the new info. Mod comes in, control C, goes to publish, control V, save. Let's not make this more complicated than it has to be.
GAME: Lara Croft and the Guardian of Light
EDIT: They have updated the game to allow online Co-op. If you do not have a second controller, you would previously not be able to obtain it. You may now play it online.
A Friend in Need 20 
Play Co-op mode
This is the easiest achievement. Start a game cooperative with a second controller and you'll automatically get this, without even playing.
You can also opt to do this online with a friend if you have online access and have downloaded the update.