The following code snippet will allow you to overwrite the meta tags of a page in DNN.

/* get the current dnn page framework object */
CDefault objPage = (CDefault)this.Page;

/* we set the meta tags of the page */
objPage.Title = "THIS IS THE META TITLE";
objPage.Description = "THIS IS THE META DESCRIPTION";
objPage.KeyWords = "THIS IS THE META KEYWORD";