The phone/tablet limits the amount of amperage needed.
So the charger's ability of amperage could be bigger than the phone's/tablet's needing, but should not less
Using a higher voltage than the 5V standard USB voltage can destroy an USB device ... and not the charger's ability to supply a higher amperage at request.
For better understanding the thinking model of electricity for beginners might help
Think of flowing water in a pipe (the phone/tablet would be the pipe).
Voltage is the pressure of the water supply, amperage is the flowing amount of water and the pipe is the electrical consumer (its electrical resistance).
The pipe determines the amount of water when the pressure is steady.
There're only two ways for increasing the flowing amount of water:
- A bigger pipe for less resistance.
- Higher pressure of the water supply.
But higher pressure might destroy the pipe.