Hi,
Here is a link which shows how to open an Acer Iconia W500. it is in Russian, but the video is self explanatory. At 3:30 minutes into the video the battery has been removed.
https://www.youtube.com/watch?v=f4QLBNk0Wxg
If you 'google' Acer Iconia W500 battery replacement there are a lot of companies that supply the battery, some even offer 'installation tools' as part of a kit, with the battery.
Hopefully this is of some help.