Window removal and repair

I have older (30 years or so) aluminum, double hung, double pane windows in need of some repair.
They have block and tackle balances with plastic end pieces. The plastic appears to by nylon, but has become brittle and broke on some.
Found a few places on the net that sell replacement parts, both balances and the plastic parts. That is, if can match them up - there are many variations.
In some cases both balances have broken pieces and window comes out easily. However if only one is broken, or if I just want to replace the plastic, I do not know how to remove the window. This would be the same, if I wanted to reglaze it.
There must be a trick to it, but have not found a picture of it on the net. Can someone point me to it. Thanks.
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