Updating an older Mac (2015) to macOS Catalina?
If I am running macOS Sierra Version 10.12.6 on my Mac (Retina 5K, 27-inch, Late 2015), can I update my software online to macOS Catalina?
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Yes. Just be aware that Catalina no longer supports 32 bit apps so you should either upgrade any 32 bit third party apps to 64 bit versions uninstall those apps by following the developer's instructions before upgrading to Catalina. Here is information about macOS 32 bit app support:
https://support.apple.com/en-us/HT208436
You can create a bootable macOS USB installer using these instructions (I think it is good to have one in case something goes wrong with the standard upgrade even though you should also have access to Internet Recovery Mode):
https://support.apple.com/en-us/HT201372
You should make sure to have a good backup of your data before attempting to upgrade to Catalina just in case something goes wrong.
FYI, you should always have frequent & regular backups, but this is even more important when using an SSD since it is impossible to recover accidentally deleted data from an SSD. Plus an SSD can fail at any time without any warning signs.