Mu is a tool that can be used to create, edit, save and upload programs to the Micro:Bit. It is slightly harder to use than the block system provided by the PXT toolset however, it will allow you to do more with the Micro:Bit and understand exactly how it works. If you want to understand the logic behind the code then learning how to write code is a must.
First go to https://codewith.mu/ and download the program. Once this is done you will have to install Mu.
If you need help using Mu go to https://codewith.mu/help/latest/ and see if your question can be answered there. The main thing to be aware of is that once your code is written use the Check button at the top of the screen to check your code for syntax errors. Make sure you correct all of these errors before uploading to the Micro:Bit. Once the code has been checked and corrected use the Flash button to upload the code to the BBC Micro:Bit.