WARNING: These steps are only to be applied if you DO NO HAVE A PMS (Reservit PMS, Hotello, IQware, ect). Otherwise, the inventories must be managed in your PMS and must not be modified as shown here. 

In the menu, go to Inventory > My planning

On the dark blue line, which corresponds to the inventories, click on the start date of your choice.

Choose the inventory to add. Caution ! This replaces the current information and doesn't count the reservations already taken. If you add an inventory of 4, you will have 4 units available in this category regardless of the existing reservations for these dates