Once you've cleaned all the leaked filament from the heater block, heat break, nozzle and heat sink, re-install the heater block on the heat break properly, along with the nozzle.

The key factor in this is that the heat break should extend into the heater block far enough that the nozzle does not sit flush on the heater block, but have a gap of one millimeter or so. You have to heat the nozzle assembly before unscrewing things and adjusting it. It might be wise in your case to separate the heater block from the heat break and the nozzle from the heater block, to ensure you have to filament remaining inside to interfere with proper assembly.

Use great care around the thermistor and heater cartridge wiring or you'll have more than one repair to address.

Once clear (and cooled), thread the heater block onto the heat break. Thread the nozzle into position until it contacts the heat break. If it is not a millimeter from the heater block, back the nozzle off a bit and tighten the heat break farther into the block. With the heat break and nozzle properly adjusted, hold securely the heater block and tighten the nozzle. This will also tighten the heat break to the heater block.

Bring the assembly up to temperature, about 250°C should do, although some recommendations state 300°C and re-tighten carefully.

This will eliminate the leaking filament. If not, something else is seriously wrong.

You should not be soldering anything other than wires on a 3D printer, unless you're using the soldering iron to smooth out plastic on a model.