Firstly, please power down the Base Station. You can format the SD card by taking the SD card out of the Base Station. See the below photo which shows the location of the SD card, which is at the back of the device, next to the ethernet cable port. To remove the SD Card, gently push the SD Card in a little way (something like a paper clip may help you do this), and this will then spring load it so it is pushed out from the Base Station when you release pressure from it.
Next, you can format the SD Card to FAT32 format with a computer (using the file explorer type of program on your computer is the quickest and easiest way). You will need to use an SD Card adaptor of some sort to do this. Many computers have an SD card slot on them these days, but you'll need a microSD to SD Card adaptor as the slot on your PC will be too large. The other way is to use a Card Reader, which connects to a USB port on a PC.
You can then put the SD card back into the Base Station (noting that the SD card goes in upside down, which means the metal contacts face upwards). To reinsert the SD card, you simply push it in gently until it won't move in any further. When you release pressure from the SD card, it will spring back out a small amount until it locks in place. Now, power the Base Station up again.
Please note, if you are going to replace the SD card, don't use an SD Card that is larger than 16GB.
If you are getting the app message "Missing SD Card" from your Base Station, then please read the full solution for this.