You will notice pockets inside the lower part of the sides of the
window.

You can get these open by raising the sashes and removing the "parting
bead" a strip of stock 6mm timber readily available from builders
yards.

Lift the first sash and pin or wedge it in place. Pull out the beads
with a painters scraper/ putty knife or a thin chisel. (It may be
pinned with 1 1/2 ovals.)

Lift the other sash and pin or wedge. pull out pocket. These are
usually pushed fitted and held in place by trhe parting bead. But some
fool may have pinned them too.

Pull out the weight and replace with heavier or with some lead fishing
eight over the top slid down over the new cord.

New cord has to be stretched before use. Tie it to wall and put your
weight on it a few times.

Use a linen string with a small strip of lead (a "mouse") to pull the
cord down over the pulleys. Tie it to the weight, cut to the same
lengths -or an half inch or so less than the original and tack to the
sash. (Having marked where the last length reached that is.)

It is not "done" to paint the sides of the windows nor the insides of
the sash. However, in a refurb, you might give it a lick of undecoat.
(Given a day to thoroughly dry, wax it with spray furniture polish or
rub with a candle, for all the maintenance it needs.

Feed the weights back in an put the pockets back and replace the
parting bead or buy new. There may be some difference in the sizes of
new stuff and that will require judicial adjustments that may have to
include pinning it.