Customizer → Fabrics → click a fabric → Overview → Default fabric toggle. The star icon in the fabrics list shows which one is current.
Exactly one fabric per account can be the default. It is the fallback when a product has no per-product default. Turning the toggle on for one fabric turns it off for the previous default.
I set a default but the storefront shows another fabric. A per-product default is set on that product, or the default fabric's rule excludes the product. Check the fabric's Products tab.
Does the default change existing quotes? No. Quotes store the fabric chosen at submission.