You must unlock your bootloader via htcdev.com (unless ofc you are on the dev edition).
You must have a custom recovery installed
Go to http://get.cm/?device=a5 and download the latest CM12.0 nightly.
Locate 5.0 Gapps via a quick Google search.
Fully wipe your device from Recovery if you are not coming from CM12.0 already.
Reboot into recovery and install.