To download the disk image safely, you should use official sources from Apple. Official Download Links
If your Apple ID downloaded Yosemite before 2018: mac os x yosemite 10.10 5 download dmg
Go to your Applications folder and double-click "Install OS X Yosemite" to begin the OS upgrade. 3. Alternative: App Store Purchases To download the disk image safely, you should
softwareupdate --fetch-full-installer --full-installer-version 10.10.5 To download the disk image safely
DMGs from unofficial sources can contain malware or keyloggers.