At the end of a season pull the plants out with the attached soil over a tray or 5 gallon bucket. Knock the soil loose from the roots and discard or compost the root/stems if they are not too woody (most veggies are annuals are are generally not woody). Take this opportunity to mix in some finished compost and/or organic soil nutrient amendment (plant food) with the used soil. Add the soil back to the pockets with new starts or seeds to give them a good start. Repeat each season.
After multiple plantings/seasons of use, it may (depending on the circumstances) be necessary to pull out all the soil and refresh it with minerals and nutrients, perhaps some more vermiculite to lighten in up, etc. However, we have towers that have been used for 3 full years and have not refreshed the soil to that extent in any of them. It’s not maintenance-free, but if everything is working well it should be very easy to keep healthy compared to conventional gardens and even typical container systems mostly devoid of soil life.