Old putty is rock hard. Soften it with a heat gun (careful not to break the glass). Scrape it out using a putty knife. Remove any metal glazing points holding the glass.
: Spray the glass with water and press bubble wrap against it to reduce heat transfer. Shrink Kits
100% Silicone (best for weatherproofing) or Paintable Silicone (if you plan to paint).