How to change 4:3 aspect ratio of slide sizes into 16:9?

I’m using LibreOffice Suit Version: on LinuxMint based on ubuntu 14.04; I would appreciate if you would mind help me through changing the default 4:3 dimension ratio of Impress Slide Size to 16:9.
Thank you!

It now (using LO 5.4) comes under Slide > Slide Properties , window ‘Page Setup’ comes up, use tab ‘Page’

Try Following steps:

1- Go under Format Tab.
2- select Page.
3- again in Page tab, under Page Format , change the format into Screen 16:9

I am trying this in LibreOffice Impress in Ubuntu and here the Format menu simply does not seem to have a Page item?!