Block explorers have two parts: The visualization front-end using a web framework and routes, such as React or Angular, and the data mining part that queries full nodes and computes statistics. I'm sure you want to modify the metrics the block explorer displays, so you'll want to focus on this section of the repo:
https://github.com/janoside/btc-rpc-explorer/tree/master/viewsThe views/ folder also contains the HTML text elements that are printed along with each metric, in case you want to modify those too.
