You don't :-) Fabric does not get restored once dyed, and the reality is, you're looking at dyeing, effectively. What you can do, if you want to hack it, is get yourself some dyes and dye-grade bleach et cetera and re-dye your shirts until you like them again. Major craft stores have the chemicals. I keep a "vat" in our basement (a galvanized tub) with a hose and an immersion heater (you do need the heat). I don't use it very much, but it's good to keep around, especially in less certain times. Some dyes and bleaches work in ordinary washing machines, but not as well, you need real heat and I don't like the idea of those chemicals wearing down our washing machine. This is not a slam-dunk or a $5 fix, it's work, but it can be lots of fun, and you never quite know exactly what you will get.