Here's a little guide:
Find the weapon of the spell. In this case, lockdown. You can either find it in the list, or go through the order/tech it is used by. Change the lockdown's explosion to whatever you want it to be, set the damage, and you're done.
Have fun.
Note: To remove the lockdown, you MUST change the lockdown's explosion setting.
I just realized that I had gone over this in the Simple DatEditing tutorial in my siggy. You might want to go there for a slightly more detailed (but a bit outdated :/) tutorial that goes over a different way to do it.