There are two scenarios asked in this question:-
1)You don't have access to the .pem file that's why you want to create a new one.
2)You have the .pem file access with you but you just want to change or create a new .pem file for some vulnerability or security purposes.
So if you lost your keys you can scroll up and see other answers. But if you just simply change your .pem file for security purposes follow the steps:-
1)Go to AWS console login and create a new .pem file from the key-pair section over there. It will automatically downloaded .pem file into your pc
2)change permission to 400 if you are using Linux/ubuntu hit the below command
chmod 400 yournewfile.pem
3)Generate RSA of the newly-downloaded file in your local machine
ssh-keygen -f yournewfile.pem -y
4)Copy the RSA code from here
5)Now SSH to your instance via previous .pem file
ssh -i oldpemfileName.pem username@ipaddress
sudo vim ~/.ssh/authorized_keys
6)Give one-two lines space and paste the copied RSA of new file here and then save the file
7)Now your new .pem file is linked with the running instance
8)If you want to disable the previous .pem file access then just edit the
sudo vim ~/.ssh/authorized_keys
file and remove or change the previous RSA from here.
Note:- Remove carefully so that newly created RSA not get changed.
In this way, you can change/connect the new .pem file with your running instance.
You can revoke access to previously generated .pem file due to security purposes.
Hope it would help!