To change a character attachment you will need to load a character first. You can do this by using menu File -> Open, or locating a character file in the Asset Explorer and double clicking on it.
Make sure that you have have the character asset entry selected. The quickest way to do so is to click on the Character button in Scene Parameters:
Now you can observe the list of existing attachments (if any) in the Properties Panel:
By expanding each of the attachments you can see its properties.
To add a new attachment you can click on the button next to the "Attachments" text (32 in our case) and use the "Insert" function.
Or ordered directly in the list:
You can see Joint attachments visualized in the viewport with blue boxes. One can be selected by clicking on it or by drawing a rectangle around it. As soon as an attachment is selected you will see it selected in the attachment list in the properties panel.
As soon as you have selected a joint attachment you can change its position and orientation in the viewport with a gizmo. To switch between translation and rotation you can use the toolbar on top of the viewport.
You can use toolbar to change between local and global transformation modes.
If you want to work in the Bind Pose you can enable it in Display Options.
Attachments can be removed through the context menu that appears when you right-click on its name in the Attachments list.