Homepage and Mobile

In homepage I use 5 blocks. Two blocks contain images with improper size for the mobile version, such as block 4 and block 5.
I've created two new block with other content that I want when a site is visited by phone to see them. Ie Block 4 and 5 to be replaced.
What code to use to change these blocks:
For example:
Desk - block 1,2,3,4 and 5.
Mobile - block 1,2,3,6 and 7.

Thanks