The Original Prusa SL1 has an update-server, which is checking for updates automatically. There is no need to do it manually, but this option is still possible.
Starting from firmware 1.5.0, you can also choose to check for beta versions of the firmware. Note that the beta versions are the ones that have not been officially released, and might have bugs still to be fixed before the final release.
If your printer is connected to the internet, it can check whether a newer firmware is available online. Go to the LCD menu -> Settings -> Firmware. There, you can 'Check for updates' and select if you wish to receive Beta updates or not.
If any updates are available, select the option 'Download now', and the new firmware will be shown to be downloaded and installed.
Starting from firmware 1.5.0, you can also easily downgrade the firmware by selecting the option in the menu. The printer will downgrade the firmware and restart. Note that you can only downgrade once from the current firmware. For example, if you have firmware 1.5.0 you can downgrade to 1.4.2, but further downgrades are not possible.
If the printer is not connected to the internet, you can update or downgrade the firmware using a USB flash drive.