If the value of the US currency in relation to the Mexican peso changed from US$1 = 20 MXN to US$1 = 19 MXN, it means that the value of the US currency has weakened or depreciated relative to the Mexican peso. In other words, the US dollar is now worth less in terms of Mexican pesos than it was previously. This change suggests that it takes more US dollars to buy the same amount of Mexican pesos, indicating a decrease in the purchasing power of the US currency in the context of Mexican currency exchange.