Most .nbu files are Nokia mobile phone backups, so the only nbu to xls conversion that would make sense is an export of contacts saved within the backup files and exporting them as Excel spreadsheet (.xls, .xlsx). For this you will need utilities, such as Noki, which were designed specially for the purposes of handling Nokia backup files.
Convert Nokia PC Suite 6.82+ backup to Microsoft Excel 97 to 2003 workbook.
We found one software available in the database for this conversion.
Microsoft Windows software - convert nbu to xls on Windows
A small usefull program for exploring and extracting of files created with Nokia PC suite
Convert from nbu file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.