This assumes you have reviewed IBM GSKit (Global Security Kit) - Getting Started.

If you did not use the "-default_cert yes" option when adding a certificate, you can set a certificate as the default certficate using the "-setdefault" option.

ihs_home/gsk8/bin/gsk8capicmd_64 \
-cert \
-setdefault \
-db /path/to/example.kdb \
-stashed or -pw your_password \
-label "certificate_name"