I want to take the phone number of site member (currently login person) and then update his/her number in the site member database, how can I achieve this?
If you are wanting to let your site members change their own telephone numbers and change it in the dataset, then you have two options.
You can either let them do it themselves by using the My Account page if you have already used the Wix Members app in your website and therefore will have had the Wix Members Collection called Members/PrivateMembersData collection added to your site automatically.
https://support.wix.com/en/article/adding-a-members-area-to-your-site
https://support.wix.com/en/article/corvid-wix-members-privatemembersdata-collection-fields
Or you can add your own Member Profile pages where the site member can go into their own profile page that you have made up and edit their own info, which again will be only available to each site member themselves, however you will be able to add and edit things in this dataset as you are creating your own Members dataset for it.
https://support.wix.com/en/article/corvid-tutorial-building-your-own-members-area