Replacing a chair base involves detaching the old base from the seat and installing a new one. To do this, gather essential tools: screwdriver, wrench, and possibly an Allen key. Remove screws or bolts securing the base to the seat, carefully lift the chair off the old base, and align the new base with the seat. Secure it by tightening the screws or bolts. Choose a base type (5-star, 4-leg, etc.) that suits the chair design and application. Consider materials like steel for strength, plastic for affordability, and wood for aesthetics.