To upload a sub-report in JasperReport Server do the following:
1) Log on to JasperServer web interface and right click on the any directory where you want to save the sub-report.
2) „Add resource” then „File” and then „JRXML”, browse sub-report and give any name.
3) Open IReport and go to main report and from palette drag „Sub report” and browse sub-report from your machine.
4) Add parameters in case you have any .
5) Select sub-report and go to property section -> „Sub report expression” property, edit and change the expression to something like „repo:/path_of_subreport_jrxml_in_JasperReport_server”