The current server certificates are stored in xampp\apache\conf
folder in their respective folders.
file to generate new certificates then rename the existing certs and replace with your newly generated certificates.
If you want something more sophisticated and still free that will not give a certificate error with your self signed certificate then here is a link to a free SSL certificate issuing site, worth a good read first as it could save you much money in regular non self signed SSL fees:https://www.cacert.org/xampp\apache\bin
contains the openssl.exe
file which will also create a self signed server certificate and also provide the files needed by the above site - simply double click and type help at the OpenSSL>
Please lets us know back if you were successful.