Your addresses change automatically every time you transact for security and privacy reasons.
However, all your previous addresses still work fine. You can manually specify a previous receiving address by clicking on "PREVIOUS ADDRESSES" link from the "receive" tab of your wallet.
Also, you can tick the "Manual receiving address" tickbox at .../Settings, which will stop the application from creating a new address automatically.
We strongly recommend to use a new address each time you transact.
Note: Ethereum and Ethereum-based coins and tokens are account based, and the receiving address does not change.