Q.How to repot an overgrown philodendron selloum when the bottom stem is so high from all the leaves that died in the past.
How can I repot it so it won’t have the long bottom trunk on it? It’s a house plant I had over 30 years.

You can tell it's time to move your Philodendron Selloum to a larger pot when the roots have filled the existing pot. The new container should be only 1"-2" wider and deeper than the previous container.
A Philodendron Selloum grows best in rich, slightly alkaline soil that retains moisture. The leaf tips of a Philodendron Selloum burn if there is too much salt in the soil usually due to over-feeding.
Prune a Philodendron Selloum with sharp pruners or scissors to control its size and shape. You can remove entire leaves be cutting them off at the base of the leaf stem. You can remove the lower leaves if you want to reveal the plant's stem. Wear gloves when pruning a Philodendron Selloum and wash your hands and tools when finished, you don't want to get the sap in your eyes or mouth.