Honda FourTrax Rancher TRX420FE1 2019 Owner's Manual page 146

Coolant
3. Remove the radiator cap lid (4) and reserve tank cap (5).
Always add coolant to the reserve tank. Do not attempt to add
coolant by removing the radiator cap.
4. Add coolant to the reserve tank as required to bring the coolant
level to the UPPER level mark.
FRONT
(4) radiator cap lid
(5) reserve tank cap
5. After adding coolant, install the parts in the reverse order of
removal.
136 Servicing Your Honda
(4)
(5)