In which case it's not a template; it's a document.
That the content you're typing reverts to the original indicates that what you're trying to do is to overtype content that is output via field coding in the document. You can't edit fields that way. What you need to do is to press Alt-F9 to expose the field codes in the document, then scroll to where the content occurs within the fields edit the content there.
Unless you know what you're doing, don't overtype any field braces (i.e. { })) or mergefield contents.
When you're done editing press Ctrl-A, F9 then Alt-F9 again to restore the field code display with the updated output.