The simplest would be to do two Replace operations. The first would clear your manual changes. The second would change everything to your desired entry.
That is, Replace "-January" with "January" then replace "January" with "-January"
Otherwise, you could use the Replace button and Next button to manually skip ones you do not need to replace.
Third option, Replace (all) "January" with "-January"
Then Replace (all) "--January" with "-January"
If you have no intentional instances of -- in your document, you could simply do the replaces for the twelve months replacing all. This will give you some with two dashes.
Replace "--" with "-" after you have done all months.
Question, are all instances of the months that you have not manually changed preceded by a single specific character? If so, you could include that in your replaces.