Hello
There is a a requirement to have a higher version of OS and Database.
As there i no indication to the database and OS the systems you have I cannot be specific here.
Check these links:
https://websmp208.sap-ag.de/erp-inst
Also check PAM and select the right OS and DB you have.
Additionally read these notes:
Note 1566412 - Enhancement package 6 for SAP ERP 6.0: Required SWC
Note 1496212 - EHP6 for SAP ERP 6.0 SP stacks - Release & Information Note
Note 774615 - Support Package levels of ERP/ECC installations/upgrades
Note 1598141 - SAP Enhancement Package 6 for SAP ERP 6.0:Compatible Add-ons
EHP upgrades are cumulative.
http://wiki.sdn.sap.com/wiki/display/EHP/EhP+FAQ
Regards
RB