Well, strictly speaking, if this is a universal prefix, you only need to put it in a form or report. Putting it in a table just takes up unnecessary space.
but, if you insist...
Create an update query. In the grid, put just the field you wish to update. in the "Update to" row put "PUBID" & [MYFIELDNAME] (substituting the name of your field for myfieldname. Sorry, but people can be so literal). Make sure the square brackets are there, otherwise the query might see the field name as text!
Run the query. Bingo!
Caution: make sure the field length is sufficient to accommodate the extra characters!
To put "0" in front of the records, do the same, but the field must be a text field, or you'll get a type conversion error.