These are the steps I noted down when flashing custom ROM to update from HTC (Android 5.0) to Lineage OS (Android 7.1). Yours should be pretty much similar. Cyanogen OS is now replaced by Lineage OS.
1. Unlock Bootloader from htcdev site for HTC using fastboot and adb
You can also get ADB files from Android SDK
a. fastboot oem get_identifier_token
b. fastboot flash unlocktoken Unlock_code.bin
Install TWRP recovery using fastboot and adb
a. fastboot flash recovery twrp-3.0.0-2-m7.img
b. fastboot erase cache
You should now have these files in your folder
3. Backup data
4. Root the phone by flashing latest SuperSU via TWRP Install. Remember you are still in old OS only now unlocked and rooted.
5. Download the custom ROM OS and openGapps (for google apps)
6. Wipe and then Flash Lineage OS followed by opengapps
Happy Android folks!
I am using for my phone for last few months and it works great, except at few times when it crashes :-)