Update Existing leads through API

Update Existing leads through API

Hi i need some help with this,

I have web forms where clients enter 4 fields that get pushed into my CRM leads module.

Then in the sales process i need them to fill in a new form which i have developed and sorted, but
through API i need to be able to update the existing leads with new data.

The problem that i am facing is how do i direct the update API to the unique Id's?

I ask this because with new leads coming in everyday how can i point to new id's that dont exist yet.
From what i have seen the unique id's do not have a numerical order so entering the id's as a list might not work.

Please help