How do I remove a step up to my Whirlpool bathtub?

You will have to replace your tile floor. All it is a step made out of 2x4s, plywood, with ceramic tile over it. Chip off tile until you expose the wood so you can remove the step.

Note: since you're replacing the floor tile, you may want to consider replacing your whirlpool tub. They only have a 5 year warranty and really start having issues after 10 years old. Most people either totally take them out or put back a nice soaker tub or air tub. Good luck with your project.

You may not have to replace the floor tile. You will have to replace tile where the step is. Remove the grout around where the step tile meets the tub base and the floor tile. This will prevent cracking the base and floor tile. Then gently start breaking/chipping the tile off the step. Once the wood base is revealed you can start dismantling it. Screws holding the plywood/cement board to 2X4 framework. Then remove the framework. You can use any tile to fill in the step area. Contrast color, close color, mosaic, etc. Hope this helps.

While I'll agree that you might as well consider replacing your jetted tub if you go to the trouble of removing and replacing your tile, if you want to keep it, do so. Just make sure you have good access to it if issues do occur. I've had mine 30 years now, with no problems, and don't want to ever give it up because it's so therapeutic. But it's harder to keep clean than an ordinary tub. I do also like the idea of keeping any of the tile you don't have to break up, and finding a good contrasting tile. Tile can be expensive and very hard to match.

sledge hammer

Yes Wooden frame with tile ontop! If you remove it without doing an damage elsewhere, you will be lucky, but will still need to replace some tiles where the step has been removed.