From the Hangar tab, select the unit and right-click to bring up the context menu. From there select "Customize => Choose refit kit..." if you are changing to a standard configuration, or "Customize => Customize in Mek Lab..." to do custom selection. One important point to note, the location (in the field, field workshop, transport bay, etc.) will affect the difficulty of changes and you may want to make a more appropriate selection before starting.
Lets say you want to go whole hog and change from an 'A' configuration to a 'B' configuration. Select the Choose refit kit... option, which will give you a dialog that lists all standard configurations and list what needs to be purchased if you don't have it. When you commit to the selection it will automatically order any missing parts and place the unit in reserve until everything arrives.
On the other hand, if you want to do something simpler but not pre-configured, like replacing the damaged SPL with a ML, then select the Customize in Mek Lab... option. This will open the selected unit in the Mek Lab tab of MekHQ, which will let you change whatever you want similar to the full MML program. As with the standalone MegaMekLab you might need to designate items as pod-mounted after you allocate them in the critical hit table for them to be considered a pod swap rather than an customization. Like choosing a standard configuration it will list anything you don't have and order them when you commit to the change.
In both cases as long as you're only changing pod-mounted equipment it will be considered a pod swap in terms of difficulty and time.
