Hey rustyg, it's best to start by making sure the machine is on a flat surface and fully powered down. Locate the hydraulic oil tank drain plug and place a suitable container beneath it before opening the plug to drain the oil.
Make sure to check the manual, it provides detailed instructions. You might also want to replace the hydraulic filters while you're at it.
I've been using a vacuum pump for draining the oil, it minimizes the mess and speeds up the process significantly.
After draining, don't forget to clean the area around the plug before refilling. Dirt entering the system can cause major problems down the line.