BOM.cs

SourceText text;
using (var stream = File.OpenRead(filePath))
{
    text = SourceText.From(stream);
}

var syntaxTree = CSharpSyntaxTree.ParseText(text);
var node = syntaxTree.GetRoot();
var newNode = /* maybe mutate the tree */
if (node != newNode)
{
    var newSyntaxTree = syntaxTree.WithRootAndOptions(newNode, syntaxTree.Options);
    using (var writer = new StreamWriter(filePath, append: false, encoding: text.Encoding))
    {
        newSyntaxTree.GetText().Write(writer);
    }
}