Depending upon the Office app your using you can read/write UserForm data to/from either behind the UserForm itself or behind the Worksheet, Document, or Presentation, etc.
Using Excel 2003 for this example:
Add two command buttons (cmdRead and cmdWrite) and two textboxes (txtRead and txtWrite).
How to read data from your worksheet and add into a textbox on your UserForm.
To write out to your Worksheet, just reverse the assignment from the UserForms control(s) to the Worksheet/Range/Cell that you want to write to.
Private Sub cmdRead_Click()
'Transfer the contents of cell A1 on Sheet1 to txtRead (UserForm)
UserForm1.txtRead.Text = Workbooks("Book1").Sheets("Sheet1").Cells(1, 1)
Private Sub cmdWrite_Click()
'Transfer the contents of txtWrite (UserForm) to Cell A1 on Sheet1
Workbooks("Book1").Sheets("Sheet1").Cells(1, 1) = UserForm1.txtWrite.Text