Koinly makes it easy to move all your data from Bitcoin.Tax.
The first step is to download all your transaction data from Bitcoin.Tax and upload it into Koinly. There are a few ways you can do that:
Bitcoin.Tax does not have any API so you will need to download your transaction data from their web interface and upload it into Koinly manually. You don't need to make any modifications to the file, Koinly will automatically map the columns so that all data is imported correctly.
Add the exchanges/wallets/services directly into Koinly and skip Bitcoin.Tax altogether. Koinly supports all exchanges that COinTracker supports so you should have no issues with this.
If you only have a few transactions and the above methods are not possible for you, you can also add the transactions manually via Koinly's web interface.
That's pretty much it!