Create CSR(Certificate signing request) from Exchange Admin Center

Create CSR (Certificate Signing Request) 

from EAC (Exchange Admin Center).

1. Open the EAC and navigate to Servers > Certificates. Select the certificate that needs to be renewed and click on Renew.
2. Enter the UNC (e.g. \\localhost\c$\cert\certrequest.req or \\localhost\c$\cert\certrequest.req) path and click OK. 
3. You will see a file certificate request. created and a file created in the server.File created.

Now you created a certificate request successfully. from EAC.

Create CSR (Certificate Signing Request) 

from EMS(Exchange Management Shell).

1. Run the following command to get the thumbprint of the certificate which is nearing expiry, or has expired.

Get-ExchangeCertificate | where $_.IsSelfSigned -eq $false} | Format-List,FriendlyName,Subject,CertificateDomains,Thumbprint,NotBefore,NotAfter

2. Run the following command to renew.

Get-ExchangeCertificate -Thumbprint <Thumbprint> | New-ExchangeCertificate -GenerateRequest -RequestFile <FilePathOrUNCPath>\<FileName>.req

You will see a file certificate request created and a file created in the server.

File created.

Now you created a certificate request successfully. from EMS.

Please check this article to complete the CSR