If you by accident deleted or want to return a deleted Element ID, this can be done by the Recycle Bin located in the Home tab in Project Manager. 

In this example 2 sandwich walls (SW119 id 325 & SW120 id 326) will be deleted and then SW119 id 325 will be returned back. In Project Manager, go to model view and mark what element you want to delete and right click with the mouse. Choose Delete ID. 

Now the element(s) should be gone. In order to get them back go to the home tab and click on Recycle Bin. 

When the Recycle Bin is opened it can look something like this. Here you can either search by scrolling and finding the deleted element or you can use the Full Text Search. Here you can type and search for every column but Insertion X, Y, Z and Date Deleted. It is best to search after the Element Id because that is unique for every element. 

In the Full Text Search field type in the element Id for SW119 which is 325 and mark the box to the left. Next press restore in the lower right end. 

The wall should now be put back into the model and the project.