Is the waste pipe a clay or cast iron one?
If so you need to be careful not to damage the pipe.
The joint could be cemented or could have a rope stuffed into it (forgotten the technical term for that bit but used to fit them that way when 'I were a lad')
Work gloves, goggles, dust mask (more to protect you from inhaling toilet germs then dust).
Flush and clean the toilet as best you can and then flush it again.
Use a small hammer and chisel to carefully crack the back of the pan near the joint to the waste pipe. This will separate the pan spigot from the rest of the pan. Then smash the pan away from the pipe with a bigger hammer and clean off any cement on the floor. That will ensure the pipe isn't damaged in the process.
Get a plastic carrier bag and stuff it full of newspapers and use it to bung the waste pipe so nothing goes down it.
Then carefully chip away the rest of the pan spigot around the cemented joint so the joint is flush with a bit of pan and cement still in the pipe socket.
If possible carefully drill some small holes in the cement joint all the way around the joint, without damaging the pipe, to give a break line. Using a hammer and chisel chip the remains of the pan spigot and cement inwards and remove the bits from the pipe. Carefully chip away any remaining cement from the pipe joint. so it is clean.
Pull the carrier bag and newspaper out of the pipe along with any remaining debris.
Fit new pan with a modern rubber gasket jointed pan connector that is suitable for the pipe. You may need to make up a mortar mix to bed the new pan onto the floor if it is a solid and uneven floor.
Use a spirit level to ensure the new pan is fitted level in both directions.
