Use the Base64 alphabet to convert "ICA=" to six-digit binary numbers. "I" is 8, which is "001000" in binary. "C" is 2, which is "000010". "A" is 0, which is "000000". Ignore the placeholder "=". Concatenate the binary numbers, and take bytes (8 bits) out of the result at a time, ignoring leftover zeroes. So, you have "001000000010000000", which becomes "00100000" and "00100000", which are both 32, the decimal representation of a space character.
Base64 encoding alphabet and explanation can be found here: https://www.rfc-editor.org/rfc/rfc4648