There are several domaims metioned in my certificate. How can I remove just one domain? I want to move it to a different server.
you can’t change a certificate. Use it max. 90 days. Ignore the domain you want to move.
Create a new certificate with this domain name, use it.
Next time renewing the old certificate, create only a certificate with n-1 domain names.
If you’re using certbot, see https://certbot.eff.org/docs/using.html#changing-a-certificate-s-domains
(@JuergenAuer is correct - you can’t, strictly speaking, modify an existing certificate, but you can replace it with a new one that has the domain names you want).