<p>I have a WiFi module that only needs two wires connection to work. These are RX and TX pins connected to Arduino or the CR-10S printer board but I don't know if there is any physical or software UARTs TX and RX pins. My goal is to add a Wifi support to the CR-10S printer. Since this is not Arduino and the pins are not labeled, it's hard to tell which TX and RX pins are not being used.</p>

<p>In the image of my motherboard below, any port or pin with line pointing to is considered as being used by the printing software so I can't used them.</p>

<p><a href="https://i.stack.imgur.com/gGGeo.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/gGGeo.jpg" alt="CR-10S printer control board"></a></p>

<p>There are still ports or pins that are not used. Can any one tell if there is a  TX and RX pin that is not being used from the image above? I need them to communicate with the printer wirelessly. </p>

You are looking for a capacitor that must be connected to Pin 4 of the LM2596.
Maybe you could provide a better picture of that area so we could see the different tracks on the board.
The LM2596 is in the center of the right side of the board (it is also labeled  with LM2596D). The pins should be counted from top to bottom (in your picture)
My guess is, the Elko you are looking for is connected to C31, and you must look for the positive pin.
In this wiring diagram Cout is the capacitor you are looking for. The SMD Parts R1, R2 and CFF should be R31, R32 and C31 in your picture.
With the corresponging measurements I would say you do not need to replace the capacitors.
In comparison to the old board your board already has the "fix" implemented.