The IPod Classic is the most well known of the 5 types of ipods, having the most customization over the bunch.
Here are the following upgrades that can be made:
Battery upgrade/replacement
color/shell swap
upgraded storage up to 2TB
Fireware update/replacement
The iPod min is similar to the iPod classic, having almost the same upgrade capabilities
Unlike the ipod classic, the nano and shuffle is not as simple to repair and has risk of breaking if worked on.
Upgrades are limited to battery upgrades.
Some like the 4th and 5th gen iPods are very risky to replace the battery and could result in permanent damage to the screen, however the screen is replaceable
To Better Identify which iPod you have, you can visit the official apple support page or the Ipod guide here