Noob here with some questions:
After you upload a new sketch to your Uno, are any old sketches you uploaded still in its memory? If so can you remove them? And how do you know when you've run out of memory space?
When you upload a new program then the old program is gone. You don't need to do anything else to remove it.
You know where you're running on memory by examining your code and seeing how much memory it uses and where.