There is no simple conversion for what you want. By most commonly accepted nomenclature, the .fna files contain genomic sequence(s), while the .faa files contain proteins that are predicted from the genomic DNA. It is not a matter of conversion - one needs to predict the genes. My advice to you is to download the .faa files from the same site where you got the .fna files. If they are not available, prokaryotic genes can be predicted using prodigal. While you should read about all the program options, a typical command to get what you want would look something like this

prodigal -i file.fna -a file.faa -o /dev/null