To fix a leaning Christmas tree, you need to start by identifying the root cause of the lean. If the tree is leaning due to a weak base, try adding some additional weight or padding to the bottom of the tree stand. If the tree is leaning due to an unbalanced tree top, try trimming some of the branches on the heavier side of the tree. Finally, make sure to regularly water your tree to prevent it from drying out and becoming unstable.