Moved to .NET SDK forum.
Need more info...
Version of VS or Dev tool/Language being used?
What version of the CR assemblies are you using? Look in for crpe32.dll, it will give us the exact version, search your PC, it's not included in the project.
Are you trying to rename the formula or alter the text within the formula, IE the actual formula?
Or are you trying to update a field within the formula text?
Try doing in CR Designer what you want to do in code and then emulate the same steps in code.
Don