Should I get rid of old mulch from last year? Or leave it and add new?

Add new. Mulch will decompose over time and mix in with your soil, providing nutrients. Unless it’s rubber mulch which I would never recommend because it off-gases and doesn’t decompose at all.

You can layer over top

Add more. It is decomposing which is why u need to add more. Unless it is moldy leave it.

leave it and make sure you have a good 4 inches will keep plants cool and weeds out

Just add more! It decomposes naturally and I just refresh every spring.

I agree. Leave the old & add new.

Just a bit of warning though..I had mulched over our small garden with wood chips a couple summers ago (along with the paper that prevents weeds) to keep weeds under control since we'd be gone awhile that summer. Last year, it was breaking down some, but thought I'd just reuse it. When I tried to plant, however, I found the soil was so wet from all the winter/spring trapped moisture, my plants didn't grow well. The soil was almost like wet clay. So depending on what your soil is like, and if you're planting new plants/seeds or just maintaining a perennial bed, just be careful! But this is more for leaving it on in the fall. But, you might want to remove it early this spring if you decide the ground will stay to wet to plant. Just my two cents!

Get new

I would dig it under and let it work into the soil, it's wonderful for amending.

You don't have to, but do make sure you don't get too deep of a layer of mulch. You also want to "stir and fluff" the old mulch before putting the new mulch on top.