One can select multiple polygons with holding control, but how to unselect an already selected polygon.

It's a bug in MED - 6.4x's MED has a fix to this.

If I select some polygons and then want to unselect some of the polygons from the selection, then how to do it.

As far as I know, just hold the control key down and click on each individual triangle you want to deselect. This does not work in 6.31, however due to a bug.

Thanks, but its not as you say.

Oh, do you mean that polygons are otherwise randomly getting selected when behind one another? That is, if you had a stack like this:

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and held the control key down while clicking in the same area that they seem to randomly be chosen? The only other option that I know of is to use the 3D view to do so. Otherwise, I'm confused on what you mean. Could you explain in better detail?

I select some polygons, say 10, and for some reason I dont want some of the polygons that have been selected. Either I restart select polygons from start or there sholud be a key-combination that would let me un-select the polygons that I dont need in my selection now.

selected polygons : ---------
un-select some : -- - - -

this might be a slection set that I might like.

Another scenario might be to select the head group and then remove eyes and mouth from the selection set.

As far as I know, all you need to do is press and hold the control key and click with the left mouse button while holding the control key. That's how it's done. If that still doesn't work, you've found a bug.

The only other method I can think of is to use the hide unselected feature. Select the area you want to select (the face part as an example) in face mode and click the "hide unselected" button. This hides everything else so it can't be selected (only the head part is shown). Narrow your selection down by selecting the parts you want to select (such as just the eyes). Click the hide unselected button again and keep repeating until you have what you need. This is the only other alternative that I know of.

MSL_MANI wanted to know, how he can DESELECT one poly (which is mistakenly selected). This is done with a workaround. Just use the undo-function (which works well for Deselecting the last few selected polys), or use "select inverse", now select the not wanted polys, the again use "select inverse" -> then it's done.

I've asked the support a while ago and got this nice hint

With CTRL you can select multiple polys

Thanks Logimator. Thats a very nice solution. Works with all editions of 3dgs. In 6.4 one can deselect while holding control+shift keys together. Thanks any way for a simpler solution.