When you share contacts with one of your friends, he can change some details in those contacts. You have two ways to accept those changes.
First way is to accept or decline selected changes manually. To do that, choose Network option from upper menu.

You will see a list of Network Updates. One of them will hold information about the change: which contact has been changed, who and when did it.

To see the details, and to accept or decline that change, click verify contact changes link. You will see a list of changes. Removed details are red, added and changed are green.
If you want to accept changes, select them by activating checkbox on the left (initially, all checkboxes are selected) and click Accept changes button. If you wish to skip some of changes, just deselect the checkboxes on the left and click Accept changes.

If you want Contails to automaticaly accept all changes made by the particular user you sharing contacts with, you can do it by selecting a label containing shared contacts and activating Automaticaly accept changes by user... checkbox, in the right column.

Alternatively, you can turn this option while you accept changes manually, by selecting checkbox named Automaticaly accept changes by user...

In order to turn auto accepting off, just deselect the same checkbox in one of the locations where you turn it on. It is important to remember, that once you agree for automatic updates, ALL the changes will be accepted without any action from your part (you'll still be informed about them).