As you can see it the images you've provided, there is usually a small tab that slots into the grooves of the zip tie in order to stop the cable undoing, some of the smaller ones used with electronics can relatively easily be undone by simply pulling either side hard, however, for more heavy duty ones, this is near impossible to do so you'll need to do something else.
If you get something small and pointy (a pin or needle but other small objects will work) and put into the block with this tab and try to pull it down with the object you are using and then it should undo very easily!
It won't damage the zip tie and it will still be useable and once you've had a couple of goes at it, it'll be very easy for you to do. Some more expensive zip ties have a small button like thing on the block which will undo the zip tie by moving this tab out of the way, so if you use them a lot you could look into investing in something like this.
Here is a link to a similar method to the one I have mentioned previously with a slight variation.