In the top left corner of your chart you will see the current chart parameters. If you are currently viewing a candlestick chart, the word "candles" will be in the chart parameters. Click this drop down and go to the pink/salmon-colored section called "Time Period". Choose your time period and at the top of the menu you will see "Line". Click this option and your chart will become a line chart.
Find out more about chart parameters in the TWS Users' Guide.