Tenants not getting along - allegations of threats

I have a duplex I self manage where the tenants are not getting along.  There has been petty stuff for a while that I've tried to stay out of.  It sounds like it reached a boiling point on Saturday night.  Lower level is saying upper level threatened them.  They said they called the police but the police never showed up - I'm very skeptical of that in this area.

Both tenants pay on time and keep the place in reasonable shape but are high maintenance in terms of the things they contact me about and their bickering.  I don't really trust either of them at this point.

Lower is saying they don't feel safe and want to move - they signed a one year lease 7/1/21 and are paying a little under market.

Upper's lease expires 9/30/21.

Although I'm not excited about the potential for a completely vacant duplex heading into fall/winter in Milwaukee, I'd kind of like to get both tenants out and start from scratch.  The location is good so I'm optimistic about finding better tenants (haven't had problems in over a decade before these two).

I'm looking for help with three questions:

1) Do I have any responsibility to call the police or address the tenants myself, and if so, what do I even say?

2) If I want to let lower out of their lease, is there a specific type of document I need to have them sign?

3) If I don't want to renew upper, do I just send a letter that says your lease ends 9/30 and you need to be out by then?

Any help would be appreciated - this has been a stressful situation