I transformed permalink choice for my wordpress pages to custom, such as this /%postname%/. Now i would like default structure back i.e. ?id= but i am unable to find where i edit permalinks to default structure??

You have to be drenched along with administrator rights - however i guess you understood that already.

So visit the usual Wordpress menu, and underneath the 'Settings' heading, click 'Permalinks'.

Near the top of the page is 'Common Settings' and under that, towards the top, click on the option 'Default'.

Keep in mind, any incoming links for your site and internal links you have put in pages or posts is going to be damaged whenever you improve your permalinks.