$ cat Test.java
public class Test {
  public static void main(String[] args) {
    System.out.println("Hello, World!");
  }
}

$ javac Test.java
$ java Test
Hello, World!
$ hexdump -C Test.class
00000000  ca fe ba be 00 00 00 34  00 1d 0a 00 06 00 0f 09  |.......4........|
00000010  00 10 00 11 08 00 12 0a  00 13 00 14 07 00 15 07  |................|
00000020  00 16 01 00 06 3c 69 6e  69 74 3e 01 00 03 28 29  |.....<init>...()|
00000030  56 01 00 04 43 6f 64 65  01 00 0f 4c 69 6e 65 4e  |V...Code...LineN|
00000040  75 6d 62 65 72 54 61 62  6c 65 01 00 04 6d 61 69  |umberTable...mai|
00000050  6e 01 00 16 28 5b 4c 6a  61 76 61 2f 6c 61 6e 67  |n...([Ljava/lang|
00000060  2f 53 74 72 69 6e 67 3b  29 56 01 00 0a 53 6f 75  |/String;)V...Sou|
00000070  72 63 65 46 69 6c 65 01  00 09 54 65 73 74 2e 6a  |rceFile...Test.j|
00000080  61 76 61 0c 00 07 00 08  07 00 17 0c 00 18 00 19  |ava.............|
00000090  01 00 0d 48 65 6c 6c 6f  2c 20 57 6f 72 6c 64 21  |...Hello, World!|
000000a0  07 00 1a 0c 00 1b 00 1c  01 00 04 54 65 73 74 01  |...........Test.|
000000b0  00 10 6a 61 76 61 2f 6c  61 6e 67 2f 4f 62 6a 65  |..java/lang/Obje|
000000c0  63 74 01 00 10 6a 61 76  61 2f 6c 61 6e 67 2f 53  |ct...java/lang/S|
000000d0  79 73 74 65 6d 01 00 03  6f 75 74 01 00 15 4c 6a  |ystem...out...Lj|
000000e0  61 76 61 2f 69 6f 2f 50  72 69 6e 74 53 74 72 65  |ava/io/PrintStre|
000000f0  61 6d 3b 01 00 13 6a 61  76 61 2f 69 6f 2f 50 72  |am;...java/io/Pr|
00000100  69 6e 74 53 74 72 65 61  6d 01 00 07 70 72 69 6e  |intStream...prin|
00000110  74 6c 6e 01 00 15 28 4c  6a 61 76 61 2f 6c 61 6e  |tln...(Ljava/lan|
00000120  67 2f 53 74 72 69 6e 67  3b 29 56 00 21 00 05 00  |g/String;)V.!...|
00000130  06 00 00 00 00 00 02 00  01 00 07 00 08 00 01 00  |................|
00000140  09 00 00 00 1d 00 01 00  01 00 00 00 05 2a b7 00  |.............*..|
00000150  01 b1 00 00 00 01 00 0a  00 00 00 06 00 01 00 00  |................|
00000160  00 01 00 09 00 0b 00 0c  00 01 00 09 00 00 00 25  |...............%|
00000170  00 02 00 01 00 00 00 09  b2 00 02 12 03 b6 00 04  |................|
00000180  b1 00 00 00 01 00 0a 00  00 00 0a 00 02 00 00 00  |................|
00000190  03 00 08 00 04 00 01 00  0d 00 00 00 02 00 0e     |...............|
0000019f

we can see that "Hello, World!" string is located at 0x93 offset. Now we will replace that string with "Hello, Earth!".

$ echo -n Hello, Earth! | dd conv=notrunc of=Test.class bs=1 seek=$((0x93))
$ java Test
Hello, Earth!

Voilà! It works!