increase a prefab speed upon reaching a certain score

hi, really sorry if this has been covered before but i am struggling to find the answer i need. firstly i am completely new to any kind of programming, other than obsolete code used in the late 70's- early 80's, yes ha i am old. Anyway i am trying to increase the speed of 6 randomly spawned prefabs when a certain score is reached. i have a public float speed set to 5f and would like this to increase by an amount every time a certain score is reached and then the speed increased again by the same amount upon reaching another set score. I hope that makes sense, any help would be greatly appreciated or if you can point me in the right direction i would be grateful. I am trying to learn this c# and believe me its not that easy at my age. Thanks in advance.