This is well documented on Blynk website:
https://github.com/blynkkk/blynk-server#blynk-server
particularly the section about what to change on the firmware side and app side:
https://github.com/blynkkk/blynk-server#app-and-sketch-changes
On the firmware side it’s just one line of change (Blynk.begin). Note that OpenGarage firmware uses Blynk.config function (because the firmware manages the WiFi itself and does not delegate Blynk to manage WiFi, but changing it to use a different server is the same as before).