You are going to need to read the documentation in the source you downloaded. Basically you apply your path and then do a
however if I remember correctly there are some interesting variations on this for myth and you will probably want to figure out what the original configure command was that was used by the mythbuntu devs to generate the package.
At this point you have a choice to make. The ideal solution is to create a new ubuntu package for yourself with your new version and then install that. This will let you avoid breaking your system next time you update. This will certainly require you to learn more about ubuntu packaging and the versioning standards.
A quick search turne dup this link https://help.ubuntu.com/ubuntu/packagingguide/C/
You could also just do a make install and then remove your original myth package however that will be a bit unpredictable and will result in failures when you attempt to upgrade.