How much older?
And do the resulting edits need to be ported back to the old version?
In general, any newer version of FM can open the binaries (.fm, .book) of any older version. Any version can open the .mif of any other version, with loss of any unsupported document feature when back-porting.
You can expect dialogs on anything sufficiently old (e.g. pre-FM8 due to Unicode, and pre-FM6 in particular).
The document structures will be converted to FM2019, so make copies, before saving in place from FM2019.
Bob's answer is correct, but I would have just said "File > Open".
For little saving your time, You can put your ".fm " files in Book and just run a "BookComponentMifWash" utility.
This will convert all ".fm " files to ".mif " & also convert your ".fm" to the newer version.
This utility is provided by FrameMaker as part of its sample scripts in "$fmHome\Samples\ScriptsAndUtilities\BookMIFWash\BookComponentMIFWash.dll", You can just register this utility in maker.ini and start using to save time converting your documents
Do make copies before doing