To upgrade your players from 1% fitness all the way to 100%, you must accumulate enough equipment to lift them into higher tiers. The Property page lists all of your items and tokens with their star ratings and totals.
From this page, you may also spend credits to merge your lower-quality equipment to raise their star ratings. This is a handy function when your better players require highly-starred equipment but you only have poorer versions of the same item.
The property steward can also sell your equipment for credits, although the price you get for them is a lot lower than their value. The penalty for selling equipment can be lessened through upgrading your property steward in the Resources area.