convert underscore to dash within a number sequence without creating a date

I am trying to convert and underscore to a dash on a large number of items that are formated as text. Example as follows.
5_394 6_534 1_71 7_201
7_201 6_535 1_76 6_541
5_411 5_427 5_410 7_212

My issue is that when I do a find and replace it converts a number of these to a date but is still formatted as text.
5-394 6-534 Jan-71 7-201
7-201 6-535 Jan-76 6-541
5-411 5-427 5-410 7-212

Can someone advise how I can convert the Underscore to the dash without this happening.