I'm a little confused by pop-up message. Does the pop-up have the word ALARM then a 4 digit number?
A pop-up would be done through the DIALOG instruction right before a PAUSE or ABORT instruction and would NOT be an alarm. You'd have to find the particular DIALOG instruction that you want to modify. The text you want to change is in a STRING variable. Dissecting the DIALOG instruction would tell you what STRING to change.
If the pop-up looks like the standard Alarm screen then the text is either:
1) Alarm 9xxx. Changed by in Management security level go to IN/OUT in the Main Menu, then I/O Alarms.
2) Alarm 8xxx. Find the SETUALM instruction and change the text in the instruction or in the STRING variable it is referencing.