# The -LiteralPath options are required if the filenames contain square brackets []

$items = Get-ChildItem -literalpath "{folder path}" -recurse
# enumerate the items array
foreach ($item in $items)
{
	# if the item is a directory, then process it.
	if ($item.Attributes -ne "Directory")
	{  
		(Get-Content -LiteralPath $item.FullName ) |
		Foreach-Object { $_ -replace '{old text}', '{new text}' } |
		Set-Content -LiteralPath $item.FullName
	}
}