Here's how you can add, remove, or change a connected social platform (Twitter/X or LinkedIn. Medium is a little different).
Press your profile picture in the top right.
Click on "Settings".
Press "Linked Accounts".
From here you can add an account by pressing the blue "+" icon, or delete one using the red "x" icon.
Changing your connected Twitter account:
Remove the currently connected account.
Go to twitter.com and sign in to the account you want to connect.
Go back to Typeshare and press the blue "+" icon to connect your Twitter account.
Adding your Medium account:
Create a "New Piece" for Medium.
Once you click Publish you'll be prompted to add Medium.
Connect from there and publish as usual!