In order to access the settings page for your Stock inventory system, go to your 'System settings', scroll down to 'Stock control' and choose this setting from that section.
This is where you can configure the kind of inventory system you will be operating under. This setting, once chosen, cannot be toggled or changed so it's important to ensure you know exactly which of the systems you want to be using going forward before selecting.
Choosing 'First in first out', means that the first stock items to be entered will also be the first ones to be taken out. Whereas 'Last in first out' assumes that the last items entered will be the first ones to be taken out. For example, if you have bought a dozen cans over a dozen days, the first you bought would be taken first from the back, in the first in, first out setting, whereas in the last in, first out option, you would be taking the cans from the front as they have been purchased more recently.