The same one you used to build them. If you're looking at the item while in building mode, the name of it should appear onscreen. The button you used to build (default is 'e' on PC) should appear on the HUD labelled as "Select". Do that and the item will move with you. You can also move it forwards and backwards with the mouse scroll wheel and up and down with the mouse scroll wheel if you hold the build/select button down (although this doesn't always work).
If you want to move items around more easily, build a small rug (anything will do, but a small rug is the most convenient way) and put the item on it. Any part of the item, as long as it's touching the rug. If you look at the rug in build mode and hold the build/select button down, it will select the rug and the item touching it and treat them as a single item. The crucial difference is that the collision detection for the whole will be based solely on the rug. That makes it possible to move the item into positions it could not normally be put in. You can, for example, overlap wall or fence sections to create a continous wall or fence even if they wouldn't usually snap together. You can place joined wall or fence sections at angles to each other. You can put wall sections inside other wall sections (e.g. to rebuild a partially collapsed wall). You can put a whole roof inside a ruined roof. You can do almost anything that way, even lay power lines through walls in order to get power inside a building. You could even build an entire building in one place and then move the whole building and put it anywhere, regardless of what's in the way (as long as it isn't in the way of the rug or whatever you're using). If you want a building up a tree, with the tree passing through the building, no problem with that technique.
I really need to try this title with mods soon.
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