how can i download a new software?

Whether you can update the iOS, and if so to which version, will depend upon which generation of the iPod Touch that you have - if it's an older iPod then you may already have the latest version that it supports.

First make sure that you have the latest version of iTunes on your computer - you can check if you are up-to-date via, on a PC, 'Help > Check For Updates', and on a Mac 'iTunes > Check For Updates'

When you have that installed, connect the iPod to your computer's iTunes and copy any purchases off the iPod to your computer via File > Transfer Purchases - you may also want to copy photos and any important documents off the iPod as well e.g. via the file sharing section at the bottom of the device's apps tab when connected to iTunes, via wifi, email, dropbox etc - they should be included in the backup, but it's best to have a copy of them outside of the backup just in case. You can then force a backup of the iPod by right-clicking the iPod 'Device' on the left-hand side of iTunes and selecting 'Backup'

Then start the update by selecting the iPod on the left-hand side of iTunes, and on the Summary tab on the right-hand side clicking the Check For Updates button.

Updating the iOS version : http://support.apple.com/kb/HT4623