If you want to use an existing microSD card on your Nintendo Switch, reformatting it is an easy matter. Insert your microSD card into an SD card adapter (or microSD card slot on your laptop if it has one), then put it in your PC. Hit Start>Computer and right-click the microSD card. Then hit format. You'll get asked if you want to reformat is, just say yes, and all the old data on your microSD card will be wiped and it'll be good as new for your Switch.