First login the system and be sure you already have pem or ppk files.
Add new key pain on aws (you can use puttygen to populate private and public key but I prefer AWS itself)
After creating private key with AWS then use puttygen to populate public key
You need to copy the public key which show on puttygen screen and please save it on a notepad.
Login the linux server after login go under .shh/ and see .ssh/authorized_keys file and add copied line into below the .ssh/authorized_keys file . So there willl be two line first line old keys second line is new key.
After adding save the file and try to login with new key and if you login with the key finalisy you can remove first(old) line from .ssh/authorized_keys.
Be carefull about line and do it slowy one by one. If you are not sure create demo instance and first test on free-tier instance.
