I have the maxi cosi car seat and had the same problem. try this: reach behind the circular attachments that bind the car seat to the stroller and feel around for a small button. i couldn't see back there but could feel it. you have to click this and remove on each side. a little bit of a pain because you probably can't click both buttons and pull up all at once, so for me one side kept slipping back to the engaged position, but eventually i got it. best of luck.
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I have the maxi cosi car seat and had the same problem. try this: reach behind the circular attachments that bind the car seat to the stroller and feel around for a small button. i couldn't see back there but could feel it. you have to click this and remove on each side. a little bit of a pain because you probably can't click both buttons and pull up all at once, so for me one side kept slipping back to the engaged position, but eventually i got it. best of luck.