I have weed eater because 24w and I would like to know how to change the string attachment to a bush blade

I am not sure that that is a good idea. A 24cc engine seems small for a brush blade. And do you have the shoulder strap for this weed eater? If not I definitely would not put on a brush blade.
But if you have all the safety equipment. You usually remove the trimmer head, and apply the blade with the adapter and tighten securely.

I miss read that, you said because 24w, and it has been a really long day and I thought it said 24cc.