If it has been in there for a long time, or the block was painted after it was installed, or someone used a ton of silicone sealer you may have to carefully scrap the excess off with a knife so it will come out easier.
Some heaters recess in the block completely and others stick out just a bit.
Either way they are a pain sometimes.
You can also screw in a bigger bolt, something like a 1/4" lag bolt with a big washer on it so you have something to pry to pop it out. It is in such an easy place to work on it.

Make sure when you reinstall the new one that it is pointed so the heating element does not touch the block when installed. Element pointed to the front or back of the engine parallel with the oil pan gasket always worked for me.