I just discovered that I have anthills in my flower beds.
What is the best way to kill them and not hurt plants?
TIA

Think of ants as termite protection. Termites fear ants.

Sprinkle cinnamon on the beds.
It won't kill them , but they will move else where.

It might depend on the type of ants. I have a similar situation that I was going to ask in one of my favorite nurseries about.

I have bricks that border my perennial bed and when I turned a few over I saw a bunch of itty bitty ants scrambling around. So I know they're not fire ants. Glad they're not fire ants! They act as if they're raspberry aka crazy ants. Interestingly, crazy ants eat fire ants. But they also eat electric wires. Don't know what they do in the garden or to people.

So anyway, different solutions for different ants.

There are ant products available in any garden center.
When I have them living in my orchid pots, I water the plant with warm water. Around 110 degrees F. The ants hate that and leave. Outside in a garden you can try hotter water poured down the holes but make sure that there aren't plants there to injure.

You can use this. Its not toxic for people and mammals and plants.
Diatomaceous Earth as its a mineral with lots of benefits.
It absorbs the wax on hard shelled ants and bugs and makes then dry out and die, It is also used on hens and henhouses when there's mites and flee attacks.
With hens sprinkle on feathers or mix with sand and make hens a sandbath in a box.