It's different in every situation.  At the very least, you need these fields:

It really depends on how accurately you can set your defaults for other fields (not always possible), whether you use miscellaneous charges, taxes, optional fields, serial numbers, etc.  You can build separate .csv files for Orders vs Order_Details, using these fields, or build a single .csv file by adding "RECTYPE" to each row of headers (both header rows go at the top), where a Rectype of 1=Orders and 2 =Order_Details on the data lines.  Or you could try exporting just these fields to get started, and then add the other info you need.

Is it possible to import updated unit price in order details

No

if you have a look in the Sage folder, there is a sub-folder called "Import Templates" - there is one for OE & OE Credit notes, plus many others

Give that a try

No, with any import to a transaction table like this, you can only add records, you can't update.  Only master tables like customers, vendors, and items can be updated.

but you can use a connector such as iMan and create a custom workflow so that you can update details via an excel file, but that's outside of the scope of this question.  Just saying, there ARE solutions.  We have custom imports for OE and PO tables to save the time of updating hundreds of lines.