Change Multiple Different Negative Numbers All Into One Specific Number?

How Do Change Multiple Different Negative Numbers All Into One Specific Number? Anither Words How do I change one number into another Number

Lets say I have list of numbers like this

-24
-19
-37
-22
-24
-42
-26
-20
-28
-23
-28
-42
-23
-53 etc.

and I want all the negative number at -21 and down to be changed into -20 and all other numbers higher then -20 or whole numbers to be left alone.

So numbers -21 -22 -23 -24 .... -36 -37 -38 .... -45 -46 etc are all changed into -20 and all other numbers like -19 -18 -17 -16 .... 0 1 2 3 4 etc are all left alone.

I tried using the filter tab and couldn't see. I tried the replace key but that took to long changing each and every number from -21 all the way back to -99.

Is there an easier way to change all negative numbers in a specific zone of numbers (-21 thru -99) into one simple number so -21 would change into -20 and -22 would change into -20 and -23 would change into -20 etc etc etc.