In this example you are not using transactions.
I don't remember exactly if the VFP 7 supports them, but, please, see the following VFP commands:

BEGIN TRANSACTION
END TRANSACTION
ROLLBACK
ISTRANSACTABLE

and also

TABLEUPDATE
TABLEREVERT
TRY...CATCH...FINALLY

>I have fallowing problem:
>
>- PC should replace some data on other PC over internet with such a code. yy is a table on a remote PC
>
>

>sele 0
>use xx  (table on local PC)
>
>sele yy (table on remote PC)
>set order to aa
>
>sele xx
>set relaton to aa into yy
>scan
>   replace yy.field with xx.field
>endscan
>

>
>Problem is how to detect crash of a internet avoiding error like "error reding/writing file" or similar? I can detect existing of disk X: (for example) before transaction, but what if internet going down in a middle of transaction? Is there a way to detect it, cancel operation and continue with a program execution?
>
>Thanks in advance.