Hi!
If I understand your question correctly, I think you want to change the boot image that computers obtain when they PXE boot. If this is the case, right click on the boot image that you want your DP to offer for all PXE requests, go to properties-->Data Source and check the box "Deploy this boot image from the PXE-enabled distribution point". Un-check the same box for the boot image you want to remove.
I hope this helps,
Rafael