You can handle this in code behind file like below. Please comment GetString("general.add") statement and replace with your string i.e "Submit".
protected void Page_Load(object sender, EventArgs e)
{
mOkButtonText = "Submit";//GetString("general.add");
mCommentSavedText = GetString("Blog.CommentEdit.CommentSaved");
// Basic control initialization
lblName.Text = GetString("Blog.CommentEdit.lblName");
lblUrl.Text = GetString("Blog.CommentEdit.lblUrl");
lblComments.Text = GetString("Blog.CommentEdit.lblComments");
btnOk.Text = mOkButtonText;
}