There is no officially supported way to convert a sheet to the format of an older version.
You can install both versions and use them side by side. That way you could open your P10 sheet and create a new P9 sheet by copying the regions from the P10 sheet to the P9 one.
Unfortunately as its a PTC software this only works one by one and not with all regions in one go. Even worse when it comes to text regions - you can't copy the whole region, but at least you can copy the text. Similar but a whole lot worse for solve blocks as you must first create an empty solve block in the P9 sheet and then copy the regions within one by one.
Needless to say that you also have to manually adjust the formatting in the P9 sheet to match that in th P10 sheet.
But users like @LucMeekes have found a way to do a conversion by manually editing the underlying XML files. Maybe he can be of help to you.
Actually a Prime file is a zipped directory made of up of a couple of xml files. You can rename a *.mcdx file to *.zip and unpack it to have a look.
