Step 1: Understand the concept of 外汇汇率 (foreign exchange rate)
外汇汇率 refers to the rate at which one currency can be exchanged for another、In the context of China, it usually refers to the exchange rate between the Chinese Renminbi (RMB or CNY) and a foreign currency, often the US dollar (USD).

Step 2: Determine the meaning of 外汇汇率下降 (decrease in foreign exchange rate)
When we say 外汇汇率下降, it means that the exchange rate between the RMB and a foreign currency (like USD) is decreasing、This implies that one unit of the foreign currency can be exchanged for fewer RMB than before.

Step 3: Interpret the effect of 外汇汇率下降 on RMB value
If the exchange rate decreases, it means that the RMB has appreciated or strengthened against the foreign currency because you need fewer RMB to buy one unit of the foreign currency、Conversely, the foreign currency has depreciated against the RMB.

Step 4: Conclude whether RMB is贬值 (devalued) or 升值 (appreciated) when 外汇汇率下降
Given that 外汇汇率下降 means one unit of foreign currency can be exchanged for fewer RMB, it indicates that the RMB has appreciated or 升值 (strengthened) relative to the foreign currency.

The final answer is: $\boxed{升值}$