How do you rename existing entities with a custom name?
Solution 1:
On 1.13+, it is:
/execute as @e[type=villager] run data merge entity @s {CustomName:"\"Bob\""}
on 1.12− it is:
/entitydata @e[type=villager] {CustomName:Bob}
On 1.13+, it is:
/execute as @e[type=villager] run data merge entity @s {CustomName:"\"Bob\""}
on 1.12− it is:
/entitydata @e[type=villager] {CustomName:Bob}