How do you replace a hard drive in white imac 2006?

Face down on soft thick cloth, removing cables first.

Then fine right screw driver along base. be care.

Hello,
My go-to for Mac repairs is iFixit.com. They have very good step by step instructions.
Step One: locate the EMC number printed on the bottom of the foot of your iMac
Step Two: http://www.ifixit.com/Device/iMac
Select either G5 or Intel and the model that matches your EMC number. A link to he hard drive replacement instructions will be on the next page. Click on it.
if you don't have another computer to view the instructions while you are working on the iMac, print them as follows:
Scroll down a bit, right above the first image, you'll see
Formats: PDF XML Embed
click PDF and print
Read through the instructions twice before you begin, make sure you have all your tools laid out, a clean place to work and a sheet of paper. Every screw you remove, put it on the paper, and note where it came from. Take your time and follow each step.

If it looks too complicated for your skill level, there might be a certified apple repair store in your area who can do it for a fee.

First Gen Intel iMac Hard Drive Replacement Part 1
I would take a look at this video to see if this is something you want to take on. These are not the easiest computers to disassemble but if your good at following directions and if you have the right tools, you should be able to do this.