I have a SQL Server Primary Database file. (.mdf)
I require a program which can open the file and let me edit some data.
Since there are many fields, it tedious to open all the records and modify it.
Would it be a good idea to do so? I can always take backup of .mdf file before playing with it, since I do not have any programming knowledge.
Download SQL Server Management Studio Express and write an update query to change the required fields.
I can almost guarantee that editing the MDF file directly is risky to your data and not supported in any way by Microsoft.
If you have no programming knowledge you should get someone who does to write the update query for you. Alternatively you could read up on basic SQL yourself. Most people can get working with simple SELECT and UPDATE statements quite quickly.
Here's a good simple introduction to the UPDATE statement.
You can't update the data in an MDF file outside of SQL Server. The file format is not disclosed, and even if you'd manage to somehow make updates in it the integrity checks would at best cause the modified tables to be marked as corrupted, at worse place the entire database offline.
There is only one tool that can open and modify MDF files: a SQL Server instance of the appropriate version, as Ash has directed you. Before doing any modification to the database, I would highly recommend making a copy of the MDF and LDF files.