To copy files between your computer and your instance you can use an FTP service like FileZilla or the command scp which stands for secure copy.
To use scp with a key pair use the following command:
$ scp -i path/to/key file/to/copy user@ec2-xx-xx-xxx-xxx.compute-1.amazonaws.com:path/to/file
To use it without a key pair, just omit the flag -i and type in the password of the user when prompted.
To copy an entire directory, add the -r recursive option:
scp -i path/to/key -r directory/to/copy user@ec2-xx-xx-xxx-xxx.compute-1.amazonaws.com:path/to/directory