How to add an uniqueidentifier column and insert values

Can any body please, give me some ideas here. I have a huge table with 189999 rows. I need to select only the middle values of it. How can I do it. I added unique identifier column. How do I insert the values in it. Please, suggest. i would really appreciate your helps. I have accepted the answers not. I did not know how to do it before. Thank you all