There are two ways to backup your OWA certificates.

1. Thru certificates MMC Snapin

Open mmc & add certificates snapin as Local machine.

Click on the certificate that you want to backup & export it

follow the wizard select export private key  & enter the password.

2. Thru IIS go to directory security tab

click on the server certificate, in the wizard select export certificate , follow the wizard to export it.

Both of the above ways can be used to import certificates to another machine or if you are rebuilding the machine after the crash.

Also exchange management cmdlet import-exchangecertificate can be used for certificate import purpose.