It's like I've gone and died to Heaven. Also, metroids.
-> Hex -> How to edit hex values
1. Download the HxD hex editor (it's free).
2. Make a back-up copy of the Super Metroid ROM you want to edit (right-click on the ROM file > "Copy" > "Paste").
3. Make sure the ROM has no header! SMILE can check/remove the header if you're not sure (in SMILE: "Tools" > "Header" > "Remove Header").
4. Open the ROM in HxD, then click "Search" > "Goto..." and make sure that only the "hex" and "begin" circles are checked.
5. For this example, we're gonna make all of Samus' beams travel slower by applying this change from the list below: 83968 - 22 97 B1 90 to EA EA EA EA
6. In the "Offset:" box in HxD's "Goto..." window, type in 83968 and click the "OK" button.
7. Look for the blinking | that lets you know where you will be typing if you press any keys.
8. Notice that the | is sitting right in front of the values 22 97 B1 90.
9. Replace 22 97 B1 90 with EA EA EA EA and click "File" > "Save"
10. That's it. You're done. Load the ROM and fire away. To undo it, goto offset 83968 again and change EA EA EA EA back to 22 97 B1 90. This method applies to every change on this page. The ones that say "to ??" mean they're adjustable, so test them out until you get what you want. Be sure to make back-ups of your ROM often and write down which hex tweaks you decide to use (trust me, it'll save you a lot of headache if some problem pops up later with one of them).