how to update a timestamp column which has unique constraint on it

I have a OEM grid control IOT table which define as follows, I found out there are some future timestamps associcated with some targets, I wanted to change them back to current date, but when I tried to update it  I found out the start_collection_timestamp should be unique, it has bunch of wrong records there, what's the best way (script) for me to update them and don't violate the unique constraint?? I need kind of loop to change each record to make sure the start_collection_timestamp has current date and unique, how can I achieve that, thanks.