On polkadot.js, click on the currently selected network on the top left. In the case below, we currently are on the Polkadot network. 

In the panel that opens, select the network you want to switch to. We're selecting Kusama. There are two nodes available and it doesn't matter which one you choose.

Select one of the nodes and then click on the Switch button.  

You are now on the Kusama network and can access your KSM accounts here.  

You can switch back to Polkadot in the same way. 

Polkadot{.js} Browser Plugin

In the browser extension, click on the gear wheel to open the Settings

Then select Kusama under "Display address format for

You will see the address format change immediately. While it started with a 1 for Polkadot (see image above) your addresses now start with a letter, which is the Kusama format. 

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