Use something like the script below. For more info check out your Photoshop JavaScript Reference pdf in your Photoshop install directory.
var dir = new Folder('/c/temp')
var files = dir.getFiles("*.psd");
for (var i = 0; i < files.length; i++) {
var doc = app.open(files[i]);
for (var j= 0; j < doc.artLayers.length; j++) {
var lyr = doc.artLayers[j];
if (lyr.kind == LayerKind.TEXT) {
var lyr = doc.artLayers[j];
lyr.textItem.contents = lyr.textItem.contents.replace("search","replace");
}
}
doc.close(SaveOptions.SAVECHANGES)
}