When you have NS Flex Weekend Vrij you can change to NS Flex Basis in your MijnNS account. To complety cancel NS Flex including Weekend Vrij you will need to contact the NS customer service via telephone, chat, Twitter of Facebook.

I have tried so hard to contact them through twitter and chat, but they seems like very busy and take a long time to reply my message. Is there another way other than contacting their customer service?

No, those 4 ways (telephone, chat, Twitter or Facebook) are the only ways. In my experience Twitter works very well, although it can take longer than the 15 minutes it says on the website. A few months ago I wanted to cancel my OV Fiets subscription and it was taken care of in 45 minutes. It helps if you send a DM on Twitter with all the information the customer service needs, like your name, address, date of birth, e-mail address and OV-chipcard number. 

does that mean if I change my subscription from NS Flex Weekend Vrij to NS Flex Basis I only pay the 34,95€ for the one month I used the weekend subscription? And is it true that I should only do this exactly 30 (or 31?) days after my NS Flex weekend Vrij ticket got valid? 

I would do so with the following link:

https://www.ns.nl/klantenservice/opzeggen-of-annuleren/abonnement-opzeggen.html 

Thanks in advance!

A flex subscription can be changed at any time after 1 month, so a subscription that started on the 26th can be changed from the 26th of the next month.

If you change to Flex Basis on exactly that date, you will not get billed. If you do it later, you will be billed for the next month but if you change to Flex Basis at e.g. the 10th of the month you will get the costs back for the 10th to the 25th of that month.

Flex Basis can be changed to a paid subscription at any time, no need to wait a month.