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I am in "preferences >input> object mode / mesh" looking for the place i can change the default settings in order to being able to select one object with LMB and deselect it with MMB. I only found the setting "(De)select all". But can´t find anything similar for only one object, thus, my changes affect all the objects that I might have in the scene.
Thanks a lot.
If you want to select with Left Mouse button instead the default Right Mouse Button you need to change this option:
The standard selection tools can be used to select and deselect a single item at a time.
By default the three common mouse buttons are used. The first button is setup as the select button. This can be changed in the input preferences to be either left 
LMB or right 
RMB. The second button is the opposing button to the select and is called the action mouse, this may cancel an operation or it may be used for tasks like the lasso select. As blender defaults to right select I will show that button here as the select button.
The third button 
MMB is usually activated by pressing the scroll wheel. This button is setup to move the 3D viewport camera.
The select is used to alter the selection. The blender manual calls this point selection. By clicking an item it will be selected and activated, that is, it will become the active item which may be identified separately from other selected objects. By selecting an item, all previously selected items will be deselected.
If you ⇧ Shift an item, it will be added to the existing list of selected items, the item that is ⇧ Shift will also become the active item, while an already selected item will stay selected and become the active item. If the active item is again ⇧ Shift then it will be deselected. Note that the item still remains as the active item, which can cause unexpected behaviour, until another item is made the active item.
There are some other ways to alter the selected items list -
Lasso select allows you to draw a selection polygon. This is activated by holding ⎈ Ctrl while dragging the action mouse around. Any item inside the drawn polygon will be added to the selection. For objects this works when the object origin is within the polygon. By using ⇧ Shift⎈ Ctrl you can draw a deselection polygon, any items within this polygon will be deselected.
Border select B allows you to drag a box around items to add them to the selected items, items partially within the box will be included. By dragging with the MMB you can deselect items within the border, while ⇧ Shift will also deselect. Only a single dragging click is used and needs to be repeated for further selection changes.
Circle select C gives you a circle around the cursor that can be used to add items to the selection while pressing LMB - note that this always uses the LMB and is not altered by the selection button option. The size of the circle can be altered with the scroll wheel or numpad + and numpad -. For objects this works when the object origin is within the circle. The MMB can be used to deselect as well as ⇧ Shift LMB. This stays active until canceled with RMB or Esc.
C to active circle select.