Right, I don't know if you were wondering how but here it is anyway.
1. Click the sun
2.Change the Mass to 0.50 suns.
3. Change the temperature to about 3000 K
4.Change the diameter to about 2 AU.
There you have turned the sun into a red giant, make sure your on edit mode or else the other planet will float away and it wont be able to show you what happens to earth in about 7.5 billion years.
Note: its important you do this in the right order.