This parting stop will need to be replaced, so you can discard the old one. Remove the inside stop moulding to pull out the lower sash. Scrape all rubbing surfaces of sash and frame clean of paint. This process will help the operation of the sliding sash. Do not use any method to remove paint that you suspect contains lead.
Most windows have an access pocket in the side of the jam.
Once both sashes are out, opening the weight pocket should be easy. Untie the sash cord and lower the weight until it appears in the opening. Grab hold and remove it. Check the operation of the existing pulleys. Make sure the wheel spins freely, lubricate if needed.