You usually don't need to make any changes in icehrm/app directory. But, If you are upgrading to v28.2.0, please replace icehrm/app/index.php also with the same file in new release.
Download IceHrmPro release file
Backup IceHrm Installation folder and database.
Delete all folders inside existing IceHrm installation path except “app” folder
Extract latest IceHrmPro release (zip) file and copy all folders inside new IceHrmPro installation into existing IceHrm installation path except “app” folder
Replace icehrm/app/icehrm.key with the same file included in new release
Download IceHrm release file
Backup IceHrm Installation folder and database.
Delete all folders inside existing IceHrm installation path except “app” folder
Extract latest IceHrmPro release (zip) file and copy all folders inside new IceHrmPro installation into existing IceHrm installation path except “app” folder