Microsoft has schedule Windows Phone8 for this year end, and it will bring lots of new changes in UI, like new Start Screen and hardware support too. Those who are using WP7 mobile phone will ever get WP8 update on their mobile phone but they do get WP7.8 update which will bring new Start screen to them.
But if you cant wait for the WP7.8 update and looking to get that new Start Screen then you can get it by installing an free app from your Windows Marketplace.
W Phone 8 will bring that new Start Screen to WP7.5 users, you can resize into the new small and large tiles, “install” apps and change theme color.
Resize tiles (just hold on to the tile until it pops out)
Install new app by tapping on the marketplace tile