I have already filed my 2017 return but want to make a mark to market election for 2018 can we amend this to my filed 2017 return ?

No, you do not need to actually amend your return to make that election.  To make the mark-to-market election for 2018 you need to attach a statement to your 2017 tax return.  There is not an actual form for making this election.  In order to make this election, since you have already e-filed, you will need to mail in a Form 8453 and attach a written statement.  The statement should include the following information:

1. That you're making an election under section 475(f);
2. The first tax year for which the election is effective; and
3. The trade or business for which you're making the election.

When you file in 2018 you will need TurboTax Self-Employed or Home and Business (desktop version) since your trading will be reported as a business.  

Please see Tax Topic 429 for more detailed information on how to make the election in 2016 effective for the tax year 2017, how to report this on your 2017return, and special requirements to be able to make this election.

https://www.irs.gov/taxtopics/tc429.html 

Even more information on making the Mark to Market election can be found here - https://www.thetaxadviser.com/issues/2010/feb/sec475mark-to-marketelection.html

A trader must make the mark-to-market election by the original due date (not including extensions) of the tax return for the year prior to the year for which the election becomes effective.  So, you have until April 17 to make this election for 2018.  

If you are all ready an established business, you can make the election on Form 3115 (changes in accounting method) and include it with the 8453.  Check off 'Other' and specify the 'Mark to Market' election.