does anyone whats the procedure to change Slough CAF form now ? is it via email or will they allow CAF form to be modified at a later date ?

You’ll need to check with them about the implications of doing this.

In a normal year changing your CAF after the deadline means they then treat your application as late, and you end up being at the bottom of the pile for allocations.

However with 11+ exam being late in many places some (but nit all) LAs have allowed parents to change their CAF without this penalty.

Before you attempt to change your CAF, make sure Slough won’t penalise you for it.

If you are a Slough resident, the booklet is on the website (if you are not a Slough resident, you need to abide by your own LA's instructions):

http://www.slough.gov.uk/downloads/secondary-school-admissions-booklet2020.pdf

Changing your preference or changing the order
You can make any changes to your application before the
closing date. Ordinarily, changes made after the closing date
will not normally be considered as an on-time application. The
resultant effect however, of Slough grammar schools delaying
the entrance selective test (11+) from September to October
because of the outbreak of coronavirus will mean that children
will not know the outcome of their entrance test until after the
deadline for submitting secondary applications (31/10/20). In
order to accommodate this exceptional circumstance, you will
be permitted/able to change your preferences or amend
your preference order up until 10 December 2020 without
your application being considered late – as long as it was
initially submitted before the national closing date.

_________________
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This is the updated link to change school preference :

https://www.slough.gov.uk/school-admiss ... ryId=20138

Once done you will get a confirmation email from the council
You can change the school preference until 10 December
Good luck for everyone