In EndNote Go to Edit > Output Styles > Open Style Manager and pick the style you are using check the style and then click Edit. You would then go to Citations > Templates > around the text you would remove the ( ) and then add [] so you would end up with [Bibliography Number] > next go to Bibliography and Layout > you would see Bibliography Number. and you would first remove the period then add the [] so you would end up with [Bibliography Number]
In EndNote go to File > Save As (EndNote will not allow you to change the existing style in EndNote but will always apply the change to a copy of the style) to save the changes.