here's one example from a rule I use regularly to modify solid body names. I tweaked it slightly for your use with iProperties.
strDescription_Original = iProperties.Value("Project", "Description")
strFindText = InputBox("Enter the text to be replaced" & vbNewLine & "Description: " & strDescription_Original, "iLogic Find-Replace", FindText)
strReplaceText = InputBox("Replace With", "iLogic Find-Replace", ReplaceText)
strDescription_New = Replace(strDescription_Original, strFindText, strReplaceText)
iProperties.Value("Project", "Description") = strDescription_New
Another one could be
strDescription_Original = iProperties.Value("Project", "Description")
strDescription_Array = strDescription_Original.Split("_")
'strDescription_ArrayCount = UBound(strDescription_Array)
strVariable1 = strDescription_Array(0)
strReplaceText = InputBox("Enter Description Field 1", "iLogic Find-Replace", "####")
strDescription_New = Replace(strDescription_Original, strVariable1, strReplaceText)
iProperties.Value("Project", "Description") = strDescription_New
MessageBox.Show("Description: " & iProperties.Value("Project", "Description"), "Title")