Manipulate on a field from a table

I have four fields in a table
a, b, c, d
I want to update only one field example ‘a’
please guide.


is this correct ?

then I want apply “sum” based on filter.
Please guide

You are not providing any details, like you code, for people to comment on to help you.
Also when you provide code put it in side code-fences like this:

print('Your code like this')