take over (a job or position)
When someone "takes over" a job, it means that the person who was doing the job before stops, and the new person starts.
We often use the phrase "take over for (someone)":
She's going to take over for you while you're out on maternity leave.
You can also say "take over as (the name of the position)":
I heard that you're going to take over as head coach starting next season. Congrats!