how do i delete dangling dimensions

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Deleting dangling dimensions?  Are you planing to re-dimension the drawing?  Did you know you can easily reattach the dimension to the part, grab the end of the dimension line and move it to where it should be attached, you may have to do it with both extension lines...

There's no reason you shouldn't be able to select them, sounds like something funny is going on with your copy of SolidWorks.  Even if you don't want to reposition them like John Stoltzfus suggested, you should be able to select them while within the sketch.  Not sure what the answer here is, but you should at least know that this isn't expected behavior (at  least not to my knowledge).

Can you post a screen shot of the dangling dimension while in sketch editing mode?  Just curious if there's something we can see to give a hint.

Welcome to the forum.  As John said, you can re-attach dangling dimensions by selecting them and then clicking on and dragging the little red box at the end of the extension line to the new reference entity.  At least that's how it's supposed to work.  You can't always re-attach them no matter how much you try, and I've also run into cases where the dangling dimension doesn't even highlight when you click on it.  I have discovered by trial and error that when that happens I can click on it anyway (although that doesn't appear to do anything) and then hit the Delete button on my keyboard.  That's always worked for me.  Then insert a new dimension to replace it.

Edit:  I see that you've gotten other responses regarding sketch dimensions.  I assumed you were referring to dangling dimensions in a Drawing.  I don't remember ever having an issue with not being able to delete dimensions in a sketch.

If you are unable to select the dangling dimension you may want to try grabbing it with a bounding box, that has worked for me in the past. You will probably get other features as well but it may be the only way to get the one you want.

When reattaching (or fixing) dangling dimensions something I recently learned is that it matters if the dimension was placed to a point or to an edge. The dimension must be reattached to the same type of feature as it was created with. If that makes sense. Select the dangling dimension and the side that is lost will have a red dot. drag that dot away and then to the new location to reattach. Sometimes I have to do this a couple of times to find the correct feature. I personally have never been able to reattach  a dimension that has lost both references though.

Take a look:

https://forum.solidworks.com/message/513956#comment-513956