PRNX is the format of the Print Document generated by the Developer Express' ExpressPrinting System. That file format can contain any arbitrary data.
You can't convert it to Excel. Excel is for storing tabular data. PRNX does not store this kind of data. There would be no concept of a direct conversion. The data you are printing may just happen to look like a table but PRNX is too generic.
DevExpress Printing System can export PDF and RTF files instead of PRNX. What I suggest you do is export an RTF document, open it in Word, then copy and paste the "table" part of the document and paste it into an Excel spreadsheet. Even this may not work, it depends on how your source "table" happens to be formatted.
A better solution would be to do it right from the beginning. If you are using custom software that uses DevExpress printing to print documents. If you want to export Excel documents, you need to build that into the application elsewhere, and not try to hack it off the printing system.
If you've just obtained a PRNX from somebody else, then you need to ask them if their software can export Excel instead.