Set dir manager password via PASS=passwordhere. Then do a search:
ldapsearch -h 127.0.0.1 -p 389 -D “cn=directory manager” -w $PASS -s sub -b “cn=config” “(objectclass=*)” | grep -B10 nsslapd-cachememsize
get dn from output
# ldapmodify -x -D “cn=directory manager” -w secret -p 389
dn: cn=database_name, cn=ldbm database, cn=plugins, cn=config
changetype: modify
replace: nsslapd-cachememsize
nsslapd-cachememsize: 20971520