Depends what it is you are trying to achieve, you could...
a.Have a form open that relates to the record you want to change the field of. But set it's visible property to False, so to all intents and purposes the form looks as though it isn't open.
Then on a button click you could set the value of the forms field like:
Forms!MyHiddenFormName!FieldToUpdate = "The New Value"
or alternatively
b. Build an update query to update the info in the other table(setting criteria to only retrieve the record(s) you need.
c. If it's a related table - pull the field through in the query and set the fields property to false so that you can update it on the form without the user seeing it.
There are three options. I'll leave it up to you to decide which it's to be.
Ian