Hi VladKud,
Thanks so much for using Apple Support Communities.
From your post, it looks like you want to change your default calendar on your iPhone. Is that correct? If so,
you've come to the right place to get help.
Try the steps here: Set up multiple calendars on iPhone
Once you click and open the link, please read through and complete the steps in the section titled "Set a default calendar."
You can set one of your calendars as the default calendar. When you add an event using Siri or other apps, it’s added to your default calendar.
1) Go to Settings > Calendar > Default Calendar.
2) Select the calendar you want to use as your default calendar.
We recommend the steps above because they will show you how to set a default calendar.
If you’re still experiencing the same issue, reach back out to the community for more help.
Thanks for using the Apple Support Communities.
Cheers!