You can not.
First of all, because there is no such thing as SQL Cell.
A custom application might hinder you in editing a value for other reasons too, but from lower level point of view (like when using SQL Management Studio), there are basics you should be aware of:
A table, and even less a query result is comparable to an excel sheet.
By default all table fields are "writable", if you have the proper rights given - on table, or
even field level[
^]. The fields might have some constraints, thus not all values might be accepted, but that is an other topic.
There are calculated fields, which are by definition read-only.
If you have a little bit more complicated query, than the result won't be editable, because there won't be any clear reverse link anymore between the result you see and the underlying table/tables. Thus editing the value in the grid could not be persisted - this will also lead to read-only cells.
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