In order to make this change of Simplify Mode, you first need to reassemble your DBSIDPR module using the new value, then, with the MUF down, you would run a DBUTLTY job on the same LPAR as the MUF using this input:
//SYSIN DD *
This function will alter the CXX to match what is in the DBSIDPR module. You should see two DB00905I messages in the DBUTLTY job - one showing the setting before the change, and then another showing the new value. It might look like this:
DB00905I - DIRECTORY (CXX) VERSION 15.0 (+14.0) LEVEL 1 DB SIMPLIFY YES SYSPLEX PLEXC1
DB00101I - STARTED JOB-DBUTSIMP NUMBER-54361 CXX=MY15MUF
DB00905I - DIRECTORY (CXX) VERSION 15.0 (+14.0) LEVEL 1 DB SIMPLIFY NO
When you start the MUF, you can verify that your SIMPLIFY_MODE value has been changed by viewing the DB00905I message there.