Minor versions of Entra Connect Sync are (mostly) installed automatically. Major upgrades need to be installed manually, so after downloading the latest version from the Microsoft download center.

When trying to download Entra Connect Sync version 2.5.3.0 the only thing that was downloaded was the DecommissionDownloadCentre.pdf file, and it stated the following:

Important Update: New Entra Connect Sync Versions on Entra Portal Only
Following up on our earlier What’s New communication, new versions of Entra Connect Sync are only available on the Microsoft Entra Connect blade within Microsoft Entra Admin Center and will no longer be released to the Microsoft Download Centre.

It took me some time, but logon to the EntraID portal (https://entra.microsoft.com) and navigate to the Entra Connect blade. Next to Version you will see the option to download the latest Entra Connect Sync version, as shown in the following screenshot:

The upgrade itself is not difficult. Download the latest version, start the MSI file, click on Upgrade and follow the wizard.

When finished, close the wizard and check the latest version of EntraID Connect in the Control Panel or using PowerShell with the following command:

PS C:\> (Get-ADSyncGlobalSettings).Parameters['Microsoft.Synchronize.ServerConfigurationVersion']
Name                   : Microsoft.Synchronize.ServerConfigurationVersion
InputType              : String
Scope                  : SynchronizationGlobal
Description            :
RegexValidationPattern :
DefaultValue           :
Value                  : 2.5.76.0
Extensible             : False
PageNumber             : 0
Intrinsic              : False
DataType               : String