Sadly there is no built-in Search & Replace functionality in Total Commander. The powerful Multi-rename option can only be used on file names.
However I've found a great plugin named 'FileFactory' by Russian author 'WhiteWind'.
The drawback is that it is a WCX (Packer) plugin, so you have to use Alt-F5, the shortcut used for creating zip files and such, and choose "ff_dummy" as packer name.
But what's cool is that you can use the Total Commander search tool, then choose 'Feed To Listbox' and apply your replacing on the resulting file list.