change the first character in file name

@dawidc21191320 

As long as you untick the "Replace all" then only the first match will be replaced.

Assuming the file names only have numbers at the beginning, you can use this regex to match only the dash after a number:

(?<=\d\d)-

 You can use this in the batch rename tool with the string substitution option and the "use regular expression" box checked.

If your filenames can also have numbers in the middle and you don't want to replace the dashes after those, it's a bit more complicated because in Regex, lookbehinds (something that looks backwards in the string without selecting those characters) need to have a fixed length. So, you're going to need multiple instances of this string substitution, each with a different number of "\d"s in them - these match individual digits.

// matches only a dash that comes after the start of the string followed by 1 digit
(?<=\A\d)-
// matches only a dash that comes after the start of the string followed by 2 digits
(?<=\A\d\d)-
// matches only a dash that comes after the start of the string followed by 3 digits
(?<=\A\d\d\d)-

// etc...

 Hope this helps!