As far as I see search and replace in more than one file is not possible inside of TeXnicCenter (searching inside of one file is, though). For that you need to change to another text editor. Searching only is possible over the menu “Search” and the menu entry “Find in Files” or by pressing Ctrl+Shift+F. It does exactly what it says: It lists the findings of a given pattern (“Find what”) inside of given files or file types (“In files/file types”), which are located in a selected folder/directory (“In” - you can select, that subfolders are searched, too, but it seems, it’s the default), no more, no less.
Text editors for Windows where I know, that, what you want, is possible (I’m sure there are more):
(All off these with syntax highlighting for TeX files)