You can take the db off the server and make the changes and then place back on the server.
I recommend putting the data in a regular field then in the startup script copy the data from regular field to the global field, then you only have to change the data in the regular field and then when the user logs in again the changes will be made.
Modifying a global field while a client of the Hosted database cannot make a permanent change to the value. Neither will any other user see the change thus made. Each client is given their own "virtual copy" of the global field. This is how global fields are intended to work. See:
If Stacey's advice on what to do doesn't do what you need, please feel free to describe in more detail what it is that you want to do with your global field and we can probably suggest an alternative approach.