You can access the Visual Editor using the Show Visual Editor button, highlighted in the following image.
Tip
You can use the workbench.editorAssociations (one of Visual Studio Code's workbench.editorAssociations) to set the Devicetree Visual Editor to show by default instead of the Text Editor when you open .dts, .dtsi, or .overlay files. You can do this for example in the following way:
"workbench.editorAssociations": {
"*.overlay": "devicetree.editor",
"*.dtsi": "devicetree.editor",
"*.dts": "devicetree.editor"
},