There're many ways to do that. For example, you can parse Expr field using ALINES() and than combine it back, replacing PAPERSIZE=yyy in the process. Or you can use AT() function to find position of the PAPERSIZE=xxx in the Expr field and replace it with PAPERSIZE=yyy.
Check mskb #251236 for some code.

>i have retrieved all the forms in a cursor
>
>i have PAPERSIZE=40 in my expr filed
>
>how to replace PAPERSIZE=40 with PAPERSIZE=156