You can activate it at any time after creating your account: go to Settings → Kits → Privacy and tick ‘I want to participate in DNA Connect and discover genetic relatives’. To deactivate it, untick the same option. You will also see a prompt in your account to activate it.
If you previously selected ‘Do not show again’, you can still activate it: click your initials in the top right-hand corner and go to Settings → Kits → Privacy.
When you deactivate it, your profile is no longer visible and no one else can connect with you, but information you have already shared with other users is not deleted.