The choice is yours as to whether you want to add or replace. The Pioneer will do everything the Combo drive does. You may want to keep it if you want to make back up copies of your Music CD's for example. This can be done quicker with two drives than one.
If you choose to do a straight swap then ensure the jumper on the back of the Pioneer is set to master and just swap it over with the old drive.
If you decide to keep the Combo drive you will need to change the jumper on that to slave and install the Pioneer as master.
The Mesh will have spare power leads and if you look at the wide cable attached to the back of the existing drive you will see it has another connector further along the cable. Plug that into the new drive.