how to have a newly added function field update for exisiting records

how to have a newly added function field update for exisiting records

Hi Support, 

I have added a function field to a system which already has more than 5000 contact records. It takes a value from a pick-single field and converts it to a number for filtering (greater than, less than etc).

How can I have that field "recalc" for all existing contacts? Default is for new contacts only to have this but that's not much use to us.

Thank you.

Nick