Stateless British child parent visa extension

[sahajveer]

Hi experts,
I am posting this behalf of my friend.
She got stateless child dependent visa with husband on the basis of stateless child 5 year.
Her child is British citizen .
Parents completed 2.5 year visa which was granted first time now they want to apply extension for next 2.5 years.
What documents they need .?
Can they both apply together or separate application .
Please send me the link for required information .
I tried to search stateless forum in immigration board I didn’t get anything
Please response me ASAP as they have got only one week visa left ,
Thanks