how to download os x 10.11

Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.

Application Compatibility

El Capitan 10.11 Compatibility information

macOS Sierra 10.12: Compatible Apps

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.

get El Capitan from the App Store