In order to replace Phone Numbers/DID's, please follow the steps below:

Note that the settings tab is visible only to users with administrative privileges
1. Click on
Settings and then
Phone Numbers:

2. Click on
Replace Numbers:

3. The numbers you currently have will be on the left under "Configured Numbers" and the new numbers you have just ordered will be on the right under "Not Configured Numbers":
4. To replace numbers, select the ones you wish to replace from your current numbers from the left column under "Configured Numbers" by clicking on them and then click on the the new one from the right column under "Not Configured Numbers" which will pair them like this (you can replace more than one number at a time) and you click on "Submit" once done: