Reduce the space bw

how can I reduce the space bw two items in flex box ?

You need to explain what you have atm and what you want it do, flex can behave differently in different circumstances

It’s hard answer your question - i mean without seeing anything code. But if your flex items is spanning more than one roll use the align- content property.
Note that if the items is only occupying a single line the align-content property won’t have any effect.
I am assuming you are trying to reduce the space vertically though.