You'll need the right tools either way for swapping fixed cogs or freewheel assemblies; they both just spin on and off of your hub with the tool. You want a chain whip for both, and then a lockring tool for fixed, or, freewheels have different tools so you need the right tool for your brand of freewheel. The chain tension can be adjusted by moving the rear wheel forward or back, but if you're making a big change you may need to adjust the chain length to get it right, so another tool possibly needed for that.

You can also consider changing the front chainring. What kind of bike do you have? What are you looking to accomplish with changing the gearing?