Assuming you are on Windows 64-bit:
Open regedit and go to:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Reallusion\iClone\5.0
Among the values are the two main content directories listed: "Custom Data" and "Template Data".
You will want to change "Template Data" to the new location. If there is enough space then it would be best to have Template Data share its path with Custom Data, just to keep things need.
There are also two shared content directories: "Shared Custom Data" and "Shared Template Data". It's is probably best to leave those where they are.
:exclamation: CAUTION 1: Before you make any changes to the registry, it is best to export the iClone 5.0 node. (Go to File ==> Export.)
:exclamation: CAUTION 2: Most content is loaded in projects and saved, so you will most likely not have problems. Videos are not, which is relevant for certain water effects, but these are mostly saved in the Shared directories. But you may run into issues.
:discuss: NOTE: If you are on an 32-bit OS the registry path would be:
HKEY_LOCAL_MACHINE\SOFTWARE\Reallusion\iClone\5.0