In Survival Game Kit V2 the main in game HUD layout is in the BP_InventoryHUD. Here you can change the location of UI elements, resize and customize how the UI looks. Keep in mind some UI elements are hidden by default to make it easier to work in the designer, you can hide and unhide in the highlighted area shown in the image below.
Selecting some widgets will show customization options in the details panel (Not all widgets have these options) using the settings you can change some basic settings about the selected widget like the image below shows.
If you want to change more about the widget then is allowed in the settings here you can select it and then select the Edit BP_CharacterEquipment (This an example name, the name will be what ever widget you curranty have selected) option like shown in the image below, this will open that widget which will allow you to change anything you want in that widget.
Please note if you do not know how widgets work then I strongly suggest watching the videos on it and making backups before making changes as you can break you project. Any changes you make past the customization settings you will have to workout your self as that is no longer SGK specific but is to do with UE4 and Widgets blueprints.