Thank you Vinny. I did take a look at that post before I posted my question.
I believe this URL from that linked post is applicable in my case, specifically section "Dependants of PBS (Tier 1 General, Tier 2) migrants"
immigration-for-family-members/dependan ... 14832.html
However it's not clear which application do I need to make to apply further? The info says if PBS Dependant applies for FLR (M) it resets the ILR clock. So I would like to avoid that happening. My partner has already spend about 4 years here in UK ( first visa as Tier 1 General from April, 2014 to April, 2015, and then PBS Dependant leave to remain from April, 2015 to Current date)