On the face of it, 2 steps
Presuming the range is cols A to D, with the lowest number > zero
just place in say, E1: =MIN(A

)
to retrieve the lowest number
Then select cols A to D, click Edit > Replace
to replace that lowest number with the desired value
(Click Replace All)
P/s: You should always post your *complete* query in the message area, not
embed half of it in the subject line, half in the message area. The subject
line is meant to contain just a brief, key summary descript of your query.