Should you change gears when driving up a steep hill on an automatic transmission 2001 Nissan Altima?

No, you generally shouldn't manually change gears in a 2001 Nissan Altima (or most modern automatics) when going uphill. The automatic transmission is designed to select the appropriate gear based on engine load and speed. Forcing it into a lower gear might actually hurt performance and engine longevity, as the engine will likely be working harder than necessary. Let the automatic transmission do its job.