To change your Reference Currency (the currency that you see prices in on StellarX) you will need to go to your Account Settings.
Your Account Settings are in the drop-down menu under the profile icon in the top right of StellarX, just to the left of Deposit and Withdraw buttons. From there navigate to the 'Preferences' tab and you should see the 'Currency' option.
From there, you can the currency you use as your base to:
- XLM (Stellar Lumens)
- USD (United States Dollar)
- EUR (European Euro)
- GBP (British Pound)
- IDR (Indonesian Rupiah)
- CNY (Chinese Renminbi)
- KRW (South Korean Won)
- mBTC (Milibitcoin 0.001 BTC)
For all the options you can find in account settings please see This Link.