What version of Ledger is currenty installed? If you have version 90B already installed but an older release than the servce pack, then the service pack is all you need - from the link that Michael provided.
If you currently have 90A or an older release installed, then indeed you will require the Upgrade to download.
Prior version upgrades currently are at the page you are looking at. I am told that current version upgrades are available from this link -
Current Upgrades