.der is a binary format, to convert to textual .crt, you can use this code in openssl.

openssl x509 -in mycert.der -inform DER -out mycert.pem

You can also learn how to convert other certificates.

Convert .Crt Certificates

Convert .crt to .Cer

Convert .crt to .jks

Convert .crt to .pem

Convert .crt to .der

Convert .crt to .p7b

Convert .crt to p12

Convert .Cer Certificates

Convert .cer to crt

Convert .cer to .jks

Convert .cer to .pem

Convert .cer to .der

Convert .cer to .p7b

Convert .cer to .p12

Convert .jks Certificates

Convert .jks to cer

Convert .jks to .pem

Convert .jks to .der

Convert .jks to .p7b

Convert .jks to .p12

Convert .jks to .crt

Convert .pem Certificates

Convert .pem to .crt

Convert .pem to .jks

Convert .pem to .cer

Convert .pem to .der

Convert .pem to .p7b

Convert .pem to .p12

Convert .der Certificates

Convert .der to .pem

Convert .der to .crt

Convert .der to .jks

Convert .der to .cer

Convert .der to .p7b

Convert .der to .p12

Convert .p7b Certificates

Convert .p7b to .der

Convert .p7b to .pem

Convert .p7b to .jks

Convert .p7b to .cer

Convert .p7b to .crt

Convert .p7b to .p12

Convert .p12 Certificates

Convert .p12 to .crt

Convert .p12 to .p7b

Convert .p12 to .der

Convert .p12 to .pem

Convert .p12 to .jks

Convert .p12 to .cer

See Also:

How to Setup Email on DigitalOcean

Chmod Command Permissions

How to Add Sudo User in Ubuntu and Linux with Root Privileges