If you do not already have the explain tables, their DDL in in EXPLAIN.DDL so go into DB2 command mode and issue the command:
db2 -tf EXPLAIN.DDL
If you are using the control centre, you navigate to the db in question and rm-click to bring up the facilities, one of which is "Explain DDL". Follow the GUI from there and you get the explain tables populated and automatically analysed, including CPU; elapsed with colour gradation and expandability (I rarely use it but for faulty indices it is brilliantly simple).
If you are NOT using the GUI, you have db2expln and dynexpln etc. Refer to the Admin guide for specific details.