For different reason we need the different format of certificates for different operating system, many of sites offering you to convert the certificates in different format online but if you have basic knowledge of command then you can do it by own by just typing simple commands
Firstly you need to installed the openssl on to your machine, see here how to install openssl for linux and centos.
Now moved to that folder in which your certificate have, use following commands as your need.
Convert x509 to PEM
openssl x509 -in cert.cer -outform PEM -out cert.pem openssl x509 -in cert.crt -outform PEM -out cert.pem
Convert PEM to DER
openssl x509 -outform der -in cert.pem -out cert.der
Convert DER to PEM
openssl x509 -inform der -in cert.der -out cert.pem
Conver PEM to CER
openssl x509 -inform PEM -in cert.pem -outform DER -out cert.cer