To remove Exchange from an SBS server during the process of decommissioning make sure all your mailboxes are moved to another server or Office365.

Open an Elevated EMS and perform the following
#Remove default Public folders
Get-PublicFolder “\” -Recurse -ResultSize:Unlimited | Remove-PublicFolder -Recurse -ErrorAction:SilentlyContinue

#Remove system Public folders
Get-PublicFolder “\Non_Ipm_Subtree” -Recurse -ResultSize:Unlimited | Remove-PublicFolder -Recurse -ErrorAction:SilentlyContinue

#Remove Offline Address Book
Get-OfflineAddressBook | Remove-OfflineAddressBook

#Remove send connectors
Get-SendConnector | Remove-SendConnector

#Remove Public Folder database (SBS 2011/Exchange 2010 Only)
Get-PublicFolderDatabase | Remove-PublicFolderDatabase

#Remove arbitration mailboxes (SBS 2011/Exchange 2010 Only)
Get-Mailbox -Arbitration | Disable-Mailbox -Arbitration DisableLastArbitrationMailboxAllowed

#Remove mailboxes
Get-Mailbox | Disable-Mailbox

Once that is complete you should be able to uninstall Exchange either through Add Remove Programs or opening an elevated command prompt and changing the directory to the Exchange installation folder (i.e. “C:\Program Files\Microsoft\Exchange Server\v14\Bin). Then running “ /uninstall”

Once that is complete you can proceed with removing AD, and decommissioning the rest of the SBS.