If you want to delete your local copy of the NEM blockchain, it is strongly recommended to shutdown the NEM software (NCC and NIS) first.

The blockchain is a database file located at:
Windows Vista/7/8: C:\Users\[YOUR-USERNAME]\nem\nis\data\
Mac: /Users/[YOUR-USERNAME]/nem/nis/data/
Linux: /home/[YOUR-USERNAME]/nem/nis/data/

After you deleted the blockchain, restart the NEM client and it will begin to download and synchronize the blockchain from scratch.
If you don't want to wait for the blockchain to download, you can follow this guide to import the developer uploaded database.

Detailed Guide for Windows

1. Open the Windows file explorer and go to (C:) and then to Users.

2. Enter your user folder:

3. Enter the "nem" folder:

4. Enter the "nis" folder:

5. Enter the "data" folder:

6. Delete the "nis5.h2" file (this is the blockchain database):