Manually. Delete the block and try to make another one that molds properly with the surrounding blocks. Also smooth things out to prevent it.
Heightmapping is kinda pointless, as you really have to go back and revise everything you just did. It will delete any architecture you've made nearby if you are not careful, and if you stop and resume later it will mess up all the heightmapping you did before. That's what happened with yours I think.. Or you copied things with a smaller gridsize than the area was designed for.
Honestly I'd delete all of the heightmapping you have done and make it again, from scratch. The effort needed to make it look good would be more than to just start over. Don't use heightmap unless you're going to use a larger gridsize. Right now it is far too jagged and choppy.
As for the rest of the map, I didn't like it. The texturing is poor and the architecture is basic. The underground has potential to be somewhat interesting, what with dodging through rooms. I may suggest ditching the top area and making the underground the entire thing. The top is boring and ugly.
I'm gonna give this 1/5. It's your first map so usually this is what you get, and obviously others may rate this higher. Keep it up!