Bittrex - Switching Buy/Sell Coins

I'm new to trading and exchanges in general, but I've found most exchanges easy to get my head around. I'm using cointree, coinspot, independent reserve, and coinbase without issue. But, I'm finding Bittrex very difficult to get my head around. The main thing I want to figure out is how to to change the buy/sell currency. It's easy enough to pick the currency I want to buy. I just click Markets -> Pick the coin. That's easy, but once I'm there, how to I pick the coin I want to buy with?

For example, I know it's possible to buy XRP with ETH, but if I pick XRP, it automatically assumes I want to buy with BTC. But, I don't! How do I change it to ETH?

One way, I've found to get around this is to google bittrex ETH to XRP. This works, but it only works on some combinations of currencies. I'm stuck...