how can I upgrade my el capitan software please? thank you

This LiNK will present various options of macOS that can be downloaded. Just make sure the Age of the computer qualifies for the version of macOS desired. Go About this Mac and it will tell what age it is. The links in following page for the downloads Has to be Opened Only in Safari FF / Chrome will not work.

Make a Time Machine Backup before attempting. Also, when upgrading one may need to update installed Applications also