Many ways to do it. One way would be buy bitcoin, wait for it to go up against the dollar, sell that bitcoin. You could do the same with euro, British pound, Swiss franc...
Just know that depending on where you are, you may have to pay taxes on the profit you make. Say you spent 100 dollars to buy Swiss franc, then sold it for 150 dollars, you'll have to pay taxes on your 50 dollars profit.
Also remember to never bet against the US in the long term. If you're very skilled you can make some money betting against any currency, but most people just lose money.
