Read the Operator Manual for the truck and follow the manufacturer's instructions.
Further, please refer to OSHA 1910.178, Final Rule, Section III (Powered Industrial Truck Hazards) - "On the other hand, when a stand-up rider truck tips over, the truck operator can exit the vehicle by simply stepping backward, perpendicular to the direction of the vehicle's fall, to avoid being crushed. In this situation, the operator usually should attempt to jump clear of the vehicle, and should be trained accordingly."
Also, please refer to ANSI/ITSDF B56.1-2018 Section 5.3.22,(e) - "These trucks are designed with open operator compartments to permit easy ingress and egress. Although there is no sure way in all circumstances to avoid injury, where possible, in the event of an imminent tipover or off the dock accident, the operator should step off and away from the truck. These actions are intended to reduce the risk of serious injury or death."

Rules of thumb...seatbelt, harness/tether, stay with lift truck, otherwise, step out, unless the reach truck is tipping over backwards. Must be taught to all students when presenting your program. Just don't tell them to jump out, because if the reach truck tipovers backwards, they will think they have to jump. Make it clear that they stay with the reach truck when tipping forward or to the sides.

I saw the aftermath of an accident some years ago where a reach truck had tipped over forwards with the mast at around 6 metres, after the load guard at the top of the forks had struck a refrigeration unit on the ceiling of a cold store, due to the operator reversing back too far from a stack before stopping to lower the forks.

This truck had a heated cab so the operator was fully enclosed, but the longitudinal momentum caused the operator to fall against the windscreen during the overturn, pushing out the entire screen, and he fell through the window frame onto the mast, luckily crawling away unhurt just before gallons of acid started to pour from the battery, a very lucky escape!!

As a general rule you should always stay inside the confines of the lifttruck operator compartment. The machine is designed in such a way to help keep the operator protected in case of falling objects or in rare cases a tipover. By trying to exit the machine during a tipover or mishap you have no idea of how the machine is going to land and by nature it is instinct to want to get clear of a tipping over machine but your only option is to try and leave the compartment in the same direction it is falling because gravity and physics make it impossible to climb out the top side when it is tipping. In almost EVERY case i have seen the operator's chances of successfully clearing the machine is very low and it ultimately ends in severe injury or death due to the machine crushing the operator between the lift and ground.
Now being a reach truck and the operator compartment is more open and the chances of the lift tipping over backwards is very low i suppose it is possible to clear the machine if it tips over sideways or forward, but you have no idea of what is going to happen with other circumstances of what happens when the machine does tip and lands, if in racks the racks may get knocked down or any other external objects that may get knocked around.
An operators best bet is to stay put and hang on with only minimal risk of injury and eliminating the possibility of being crushed by the machine.
Now being that the redesign to alot of reach machines now a days that do include a seat, i would think that the intent is to keep you in place aside from comfort and ease of operation. So with a standup style reach truck i think it could go either way, but in the case of todays machines that include the sitdown environment the intent is to stay in place.

Having read several accident reports where a Reach truck has toppled I would recommend grabbing the overhead load guard above your head and try to stay in the seat. I have seen pictures of the operator beneath a capsized reach truck with battery acid spilling over the operator.

Reach trucks are not fitted with seat belts as the risk of trapping the operator during a lateral overturn is deemed to be lower, due to the position of the operator who is sitting sideways at the rear of the machine, hence jumping or falling from the machine would not place the operator between the truck and the ground.compared to the obvious risk with a counterbalance machine.

It is not impossible though, for example if the truck turns over backwards in a longitudinal axis the operator would still be at great risk.