From the main menu, click Ownership and select Cap table from the drop down menu.

Navigate to Shareholders from the top menu.

In the top right corner, click Actions

Select Convert from the dropdown menu. ​

Choose the relevant Shareholder, the Share Type you are converting from and to (if you haven't created the new share class, you can do so by following the instructions here), the Date of Conversion, Number of Shares and Split Factor (if 1:1 simply enter 1 in this field).

Click Convert to finalize.