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This info is taken from the USA version of the VFS visa portal for the UK - I assume the rules would be the same for India, but I don't know for sure.
Your active appointment. PLEASE NOTE, on the day of your appointment your details will be sent to our Visa Application Centre team and will no longer appear on this page. Please ensure that you upload your documents and print out details of your appointment 24 hours before you attend the centre.
(emphasis mine)
It's not obvious from this message which time zone would be used to determine when 'the day of your appointment' begins (your time, the interview/biometrics center time, the actual visa processing office, UK time, ...) - so it's probably best to follow the instruction above and ensure that all your documents are uploaded at least 24 hours before your appointment - just to be safe.
You can continue to change or upload new documents well after you've actually selected and booked the appointment date (assuming you're booking an appointment well in advance anyway).
I applied for a UK visa towards the end of last year, had my biometrics appointment earlier this month, and I uploaded additional documents 2 days prior to my appointment with no trouble.
For anyone interested: I applied for a standard visitor visa, 10-year validity, from the USA, as a non-US citizen, with UK spouse & previous visits to the UK.
Processing time was 8 days, and my application was successful.