====== Disabling weak key exchange algorithms ======

Add this line to /etc/ssh/sshd_config 


''KexAlgorithms  curve25519-sha256,curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521
''

Then 

''sshd -T  
''


Then 

''systemctl restart sshd
''