You need to cut away the soft rotten timber- then let it dry. Then apply an epoxy wood hardener- they are two part products that you mix in a tub, using a throw away brush to apply them.
In theory, you could back fill the lot with a suitable epoxy resin filler but that would be incredibly expensive.
The more common method is to "splice" in a bit of timber and "glue" it with epoxy resin filler. There is no point gluing new timber to rotten timber, the join will fail.
If done properly, it should last a "lifetime".
Ultimately, if you want to do it properly, you need to undo the screws in the hinges so that you can place the window on a work surface so that you can splice it and then fill it.
The two brands most widely available are Timbabuild and RepairCare.
Personally, I would recommend the one hour curing filler from Timbabuild
https://www.chemfix.co.uk/products/...imbabuild-epoxy-wood-repair/timbabuild-erc10/
Unfortunately, a tube will cost +£30and then you will need to spend about £30 for the gun to go with it. On top of that you will need to pay £15 for the epoxy wood hardener. (plus the cost of the small bit of timber).
If you don't want to pay that kind of money, then go down the Stixall path but please do not use the powdered Polyfiller type fillers. They will fail after a year or two. At a minimum, use the 2K fillers. They are two part fillers that you mix. You add a catalyst to the filler which causes it to harden chemically. The thicker the area to filled, the faster they cure. A 4cm lump in the middle of summer can be sanded after about 30 minutes. Unlike the powder fillers, they don't degrade if wet.