I'm hoping someone can point me in the right direction for renewing a lets encrypt exchange certificate.
I'm afraid I am quite new to this and did not setup the original certificate, so I'm not sure how it was requested and may need some handholding
Most Microsoft Exchange installs are either using win-acme or Certify The Web (because these natively produce a PFX and they're designed specifically to run on Windows). I'd guess win-acme. See if you have a C:\ProgramData\win-acme folder or if not see if you have C:\ProgramDat\certify - there's also the possibility it's using a powershell tool like Posh-ACME or PS-ACME as a scheduled task.
Sometime certs are set up manually, in which case you can pretty much start again.