The S5 switch is located underneath the PPU chip socket and has two possible settings:

REG — This is how the switch should be set by default for all PPU chips except for those listed below.

2C05 — This position should be used only if you're installing an RC2C05-01, RC2C05-02, RC2C05-03, or RC2C05-04 chip.

Note that an RC2C05-99 chip should have this switch set to REG.

If the S5 switch is incorrectly configured, no damage will occur, but the video output from your system will be a solid color or appear scrambled, and you will have to remove the PPU in order to reconfigure it.

S5 switch configuration information can also be found in Table 4: Supported PPUs in the TinyNES user manual.