Reinstall the Yellow Tree in Pogo Merge ?

As far as I know, there is not a way to reinstall items that have been deleted. There is usually an undo button immediately after deletion before you click on anything else, but that is the extent of it. You will have to recreate the tree starting with the acorns you win from completing tasks. Fortunately, the acorn seems to be one of the more popular options that they give out at the end of levels.