I purchased a 16oz bottle of slime. (https://www.amazon.com/dp/B003V9XYNW?psc=1&ref=ppx_yo2ov_dt_b_product_details)
It comes with the tool in the cap to remove the valve stem insides. I let all the air out so the tube was no longer pressurized and then you just put the provided tube over the valve stem and squeeze half the bottle in one tire and half in the other. After sliming the tire I put the valve stem back in and filled it with air before spinning it around to evenly coat the inside of the tube.
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Regarding increasing the top speed, all you need to do is hold the up and down arrow on the controller for a few seconds and then it takes you to the secret menu where you will need to increase the top speed to 40 kph.