alternative id_rsa and id_rsa.pub

copy ssh key to ~/.ssh folder:

cat Downloads/ssh-key-xyz-alt.key > .ssh/id_rsa_alt
cat Downloads/ssh-key-xyz-alt.key.pub > .ssh/id_rsa_alt.pub 

change permissions:

 sudo chmod 600 ~/.ssh/id_rsa_alt
 
 sudo chmod 600 ~/.ssh/id_rsa_alt.pub

add config-file for alternate host:

nano ~/.ssh/config

insert:

##Alternate Host
  Host 11.11.11.11
  IdentityFile ~/.ssh/id_rsa_alt

connect to remote server with user ubuntu:

ssh 11.11.11.11 -l ubuntu