There are many ways
1. If you have numbers in Col A starting at A1
enter this in B1
=Date(1,A1,1)
and format the column as
Custom|mmmm
This will show January, ... depending upon the number in A1
Or you can have
=LOOKUP(A1,{1,2,...12},{"January","February",...""December"})
Fill the missing numbers, months
or
you can use VLOOLKUP
=VLOOKUP(A1,Sheet2!A:B,2,False)
if you have numbers in Col A and month names in Col B of Sheet2
or...