There's a String constructor that accepts a C++ char (.NET SByte), so:

char* the_text = "Some ordinary string here."
label1->Text = gcnew String(the_text);

However, that's going to do an ASCII->Unicode conversion.  You may be
happier using .NET Char (C++ wchar_t) arrays.  To do that you have to write
your string literals like L"string" and character literals like L'm'.

Getting to individual (Unicode) characters of .NET Strings is easy:

interior_ptr<wchar_t> unistr = PtrToStringChars(label->Text);

To do the simple replacement you mentioned, you might do:

array<wchar_t>^ s = label->Text->ToCharArray();
if (s[2] == L'k') s[2] == L'm';
label->Text = new String(s);

You don't say what environment you are using?  Is this managed or unmanaged
code?  If managed, look at Ben Voigt's reply.  If not, you are dealing with
window handles.  Now, are you doing raw C++, ATL, WTL, or MFC?  If the former,
investigate SetWindowText(hwnd, "here is a string").

/steveA