The main menu (`Welcome to Monero` screen) will open. At the bottom left, click on `Change wallet mode` button, and on the next page select `Advanced mode`. Next, open your wallet file again.
First, you will need to find a public remote node to connect to. The website [moneroworld.com](https://moneroworld.com/#nodes) has some great resources for finding nodes. One of the easiest methods would be to use a public remote node run by moneroworld, but they have a tool for finding random nodes too.