Web Form Formula
There is a option in edit form options - Web Form Formula.
This can be used to update a user's custom field value by a constant or answer of current web form (it is highly recommended that users enable the tracking changes option of the custom field).
Example :
@#18230#@=@#18230#@+15
Add 15 to customfield(18230) whenever the user fills current web form.
@#18231#@='@form.cellphone@'
update customfield 18231 with a question, and variable name "cellphone" (variable name must be assigned to a question).
if question type is a combo box like "1=Yes;2=No"
@form.variable.text@ will be replaced with Yes or NO
@form.variable@ or @form.variable.value@ will be replaced with 1 or 2