Hi,
Please find below the link to Install/Upgrade guide which talks in detail about upgrading from 10.x to 10.6. Please start looking from the section "Unified CCX Upgrade Scenarios"
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cust_contact/contact_center/crs/express_10_6/install/guide/UCCX_BK_C2956BDB_00_uccx-install-upgrade-guide-106.pdf
Also feel free to go through the below support forum link wherein I explained step by step on upgrading from 8.5(1) to 8.5(1) SU4 that tells more on the commands and usage of free ftpd which is required during the upgrade:
https://supportforums.cisco.com/discussion/12510451/upgrade-uccx-851-uccx-851-su4
Regards
Deepak