The pipe then should be loose enough to be easily removed.
Once the stock y-pipe is removed, you will need to remove the 2 graphite seals that are on the remaining header pipes (if they are on the header pipes). They are unnecessary due to the close fit of the new Y-pipe from Remus. If the seals are inside the old H-pipe that it's you're lucky day -
I removed them with a pair of needle nose pliers.
Also, remove both of the clamps as they will not be needed and won't clamp down tight enough even if you wanted to use them.