Several ways depending on what you ultimately want to achieve:

1.
Delete the extension. 
When deleting a pop up will ask you if you want to keep the number in inventory or not.
Users > find ext. you want to delete > Checkbox > Delete

2.
Keep the extension, but delete its license
Phones & devices > Users phones > click on ext > Remove line
System will ask you if you want to keep the number in inventory or not.

3.
Keep the extension & keep its license
Phones & devices > Users phones > click on ext > Edit number > select a different number from your inventory to be associated with the license
System will ask you if you want to keep the old number in inventory or not.