You specify which slot you want to save in in the save action. I was just suggesting overwriting as there is no delete or clear slot action. You can try looking up tutorials about web storage as it does have actions to clear values and such. I personally haven't experimented with it, so I can't provide any concrete answer because I'll probably say something wrong.
Here's some helpful links abaout web storage:
https://www.scirra.com/tutorials/266/we ... nformation
Webstorage for saving values like High Scores, I doubt it can save the full state of the game. This is also off-topic.
Or just update data on that slot to empty figures
You mean change the slot? Or what? "update data on that slot to empty figures", what does it even mean? With Save Slots you have the name of the slot and actions which either save or load it.
If you're saying to change the slot, how would anybody do that in-game without access to the engine.
And if it was possible, every time the slot get changed, storage in the HDD is being wasted since it's not getting deleted.
No point in replying unless you know how to literally delete the data on a slot, since it's what I was wondering and the thread topic.