Here's commands:-
[Replace fileext with Extension of file of which you need to remove character]
1. If filenames don't contain square bracket:-
get-childitem *.fileext | foreach { rename-item $_ $_.Name.Replace("old","new") }
2.For Square brackets :-
get-childitem *.fileext | ForEach { Move-Item -LiteralPath $_.name $_.name.Replace("old","new")}