copy (save) expression value to a table field

[1aae]

Thank you
I have form and subform:
subform conatin textbox record source =expression
this expression give me serial number (1 2 3 4 5 for every record without
any effort from me) ok
example
textexpression fieldinatable
1
2
3
4
My question is:
How copy this value of textexpression to field in a table with click by
using command button in a mainform..
or my question in another way:
is there any way or event i can use with textexpression to move the value
for every record to the (Fieldinthetable)?
Notes:
I have asked this question 6 times ? until now i don't have the answer!