I don't recommend replacing the nozzle. It's a very strong part and you are unlikely to damage it. If you cant get the sensor out and you don't mind destroying the sensor you can drill it out, or use a wood screw, insert it partly in and then pull it out with pliers. Or you can drill from the opposite side a smaller hole and then push it out with a paper clip.
You can build your own sensor with any pt100 part but if you use solder make sure you use high temperature (all lead 300C) solder and not normal (200C melting point) solder.
As far as the other end of the sensor - remove the larger cover underneath the UM2. It's very easy to remove (in fact the entire UM2 is very easy to take apart and put back together - it is designed to be hacked). The temp sensor wire is very distinct - it is a white braided cable with two conductors. And it is connected to I believe "temp 1" or some similar name.
You can have a look at this:
And download the build manual for the um2 which is very usefull (everybody should have this downloaded)
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