How to modify or append on Field Symbol?

Currently I hv <FS> where in <FS> there are 10 fieldnames, I need to make calculation of (Fieldname 2 + fieldname 4 + fieldname 5) and update the result in fieldname 10.

I tried on traditional method (loop <fs> and update fieldname 10...) which it doesnt allow me to do so.

Anyone have any idea on this?

Thanks in advance,

Eric