public class Foo {
  int foo = 1;
  
  public int foo(int foo) {
      return foo + this.foo;
  }
  
  public int getFoo() {
    return foo;
  }
}

ICompilationUnit cu = member.getCompilationUnit();

if (cu.isWorkingCopy()) {
  IBuffer buffer = cu.getBuffer();
  
  IScanner scanner =
    ToolFactory.createScanner(false, false, false, false);
  scanner.setSource(buffer.getCharacters());
  ISourceRange sr = member.getSourceRange();
  scanner.resetTo(
    sr.getOffset(),
    sr.getOffset() + sr.getLength() - 1);

  int token = scanner.getNextToken();
  while (token != ITerminalSymbols.TokenNameEOF
      && token != ITerminalSymbols.TokenNameLPAREN)
    token = scanner.getNextToken();

    if (token == ITerminalSymbols.TokenNamePUBLIC) {
      buffer.replace(
        scanner.getCurrentTokenStartPosition(),
        scanner.getCurrentTokenEndPosition(),
        scanner.getCurrentTokenStartPosition() + 1,
        "private");
      break;
    }
  }
  cu.reconcile();
}