10 year route to settlement application

Hi all

I'm not sure if this is the right forum but here goes...

My partner was granted a 30 month visa under what was article 8 (but had changed to whatever it is now when his visa was granted) on the 10 year route to settlement. It runs out in May so we need to submit a new application soon. We went through a solicitor last time but we're hoping to do it alone this time, questions:

1. What application do we need to fill out?
2. When do we send it (28 days before it runs out)?
3. What is the 5 year route to settlement? What are the requirements? (I've seen a post on here about changing over from 10 year route to 5 year route)

Thanks in advance
Lea