It seems that up until version 2003 Microsoft Excel software was capable of opening of .wk5 spreadsheet files and saving them as Excel spreadsheets/workbooks. Unfortunately, newer versions do not seem to support old Lotus 1-2-3 formats any longer.
If you don't have access to Microsoft Excel 2003, try to use for some open source alternatives, some of them may still support old Lotus 1-2-3 formats. Corel Quattro Pro might still allow wk5 to xlsx conversion for example.