Identify what created the .sav file
Open it with the original software if possible
Use SPSS for SPSS .sav files
Use compatible statistical software such as PSPP, R, or Python libraries
Use a file converter if you need another format
Rename the file only if you know the actual format inside
Check the file source or documentation for the correct program
Try a text editor only to inspect the file header
Avoid opening unknown .sav files from untrusted sources