Replacing the hard drive in the eMac is not easy at all and is not considered a CIP - "customer installable part". It has been years since Ive had to work on one of these but from what I remember you have to take the entire eMac apart. I do not recommend that you do this unless you have some experience working on them. Apple recommends discharging the CRT as it can shock you and/or implode and is dangerous to deal with.
Here is some info that I found:
http://www.everymac.com/systems/apple/emac/faq/emac-replace-or-upgrade-hard-drive-expansion.html
Hope this helps!