How to write ultra speed cd-rw at a lower speed?

I didn’t know better and bought a 5 pack of memorex ultra speed
16x-24x cd-rw’s. However the first burn on my 2500a already fails
to be read. I tried to use a lower speed but Nero only allows 16x,
how can I change this?

You can’t change this!

The dyes of Ultra-Speed CD-RW are specially designed for the higher speeds and don’t work otherwise.
Sorry to say, but if they don’t work with the NEC, you need another burner for them…
I’m getting very good results with Verbatim US-CD-RWs and my ND-2500A. CD-R and CD-RW writing strategies are the same with every firmware available for the ND-2xx0A.