Power Query doesn't really do row context, but what you can do is add an index column, then load the column of values into a list that you can then access by position using the index column - 1 - for example:
let Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
#"Added Index" = Table.AddIndexColumn(Source, "Index", 0, 1, Int64.Type),
PreviousRows = List.Buffer(#"Added Index"[NAV]),
Final = Table.AddColumn(#"Added Index", "Diff", each if [Index] > 0 then [NAV] / PreviousRows{[Index] - 1} - 1 else null, Percentage.Type)
in
Final