Generally all interactions in Qlik Sense are driven through the open Qlik Sense APIs. For export I looked at Export the data of a generic object ‒ Qlik Sense Developers, which shows that you can not control the sheet name. The conclusion from that is that you also wont be able to alter the name from the within Qlik Sense.
If you for some reason need a static sheet name, keep in mind that the export will always be the only sheet in the exported workbook. This sheet will always have the default Excel sheet name when opened.
For more advanced export and formatting, you might want to take a look at Qlik NPrinting.