IWEC were completed in 2001 and are available in EPW format through a license that DOE has with ASHRAE. There are 227 locations outside the US and Canada in the IWEC, derived from weather data for 1992-1999. The IWEC are no longer available from ASHRAE.

IWEC2 were completed in 2011 and are available through ASHRAE in source format (not formatted for a specific program). There are 3012 locations outside the US and Canada, derived from weather data for 12-25 years (roughly 1983-2008). A DVD of the complete IWEC2 data set is available from ASHRAE (https://www.ashrae.org/resources--publications/bookstore/iwec2). There is also information on that web page for purchasing individual files from a third party.

Both data sets are of high quality. IWEC2 provides newer data (and 10x the number of locations).

Thanks Dr Crawley for your help. It looks like White Box Technologies prepared different formats from IWEC 2, including epw, (http://ashrae.whiteboxtechnologies.com/IWEC2-spec-sets).  

But Do we need to repeat all building energy simulations with the new sets of data (IWEC 2)  to achieve better results?

The building energy simulation results will change. I would run some tests -- using a building model and run it on both IWEC and IWEC2 and compare the energy use.

Comparing the IWEC and IWEC2: CDD (base 10C), IWEC2 is 1.4% higher; HDD (base 18C), IWEC2 is 1.3% lower; and IWEC2 has 3% more CDH (base 20 C). From this I would expect slightly more cooling.

I ran two simulation using IWEC and  IWEC2 , for a two floor building (conditioned area 280 m2), the change in heating energy consumption is -7% and in cooling is 9%. But the change in total annual energy consumption is 1.5%.