Please check our support articles and videos about how to replace an existing controller. This one is particularly relevant:
https://opensprinkler.freshdesk.com/solution/categories/5000022938/folders/5000099521/articles/5000381679-video-how-to-install-opensprinkler
The user manual has more textual details:
https://opensprinkler.freshdesk.com/solution/categories/5000022938/folders/5000147083/articles/5000632569-user-manual-firmware-2-1-5-current-