Currently the editor (ForgeEdit) is released as a beta that you have to opt into in Steam. The editor beta has the same D20 Dungeons with all your save files plus ForgeEdit. If you wish to switch back to the regular build you follows the same instructions but select 'None' for the beta in the drop-down during the last step.
Here are detailed instructions to install:
1) Open Steam.
2) Find D20 Dungeons in your Steam library.
3) Right click on D20 Dungeons to get the popup.
4) Click on BETAS.
5) Click the drop-down to get the list of betas.
6) Click on the editor