If your car seat is properly installed, it should not be rolling over at all. There should be no give in the vehicle strap that allows the seat to come loose enough for the seat to tip.
Your problem may be coming from not switch your seatbelt over to Automatic Locking Retractor (ALR) mode from Emergency Locking Retractor (ELR) mode. To do this, after you've threaded the belt through the seat and buckled it, extend the remainder of the the belt from the shoulder. Then, feed the belt back in. If you need it to be tighter than how it automatically feeds, pull up from the buckle.
If you still have issues, your car seat model should have included a locking clip. See the seat's manual for proper installation instructions, but also note the manual says that the locking clip may not solve all issues.
If you're unable to install the seat properly in your car with reasonable ease and assurance of safety, then you need to return that model and/or find a new car seat.
While I was able to find specific information for your products, this answer should still be useful to a wide variety of users, as the types of parts your products use are fairly common. That is, ALS seatbelts, rear-facing car seats where LATCH isn't possible, etc.
With that in mind, let's look at the user guide for your model of car seat (Safety 1st Guide 65):
• Switchable retractor
These vehicle belts switch from ELR to ALR (Automatic
Locking Retractor). You must have the vehicle belts in the
locking mode to use them. Follow the instructions in the
• ALR (Automatic Locking Retractor)
These vehicle belts lock after pulling the webbing out and
letting the vehicle belt go back into the retractor an inch or
two. Follow the instructions in the installation sections.
Vehicle Latch Plates
• Check the Latch Plates
Grasp the tongue portion of
the latch plate (the part that
goes inside the buckle) and
pull straight out, inline with
the vehicle lap belt. If the
latch plate does not slide
up and down the vehicle
belt, you have a locking
latch plate. You should
not need to use a locking
clip. Consult your vehicle
If the latch plate slides up and down on the vehicle belt, and you
have an ELR retractor, you MUST use a locking clip to install the
child restraint. You may not need to use the locking clip with a
switchable or ALR retractor. Consult your vehicle owner’s manual.
And here's the information from your 2013 Prius C Owner's Manual:
Installing child restraints using a seat belt (child restraint lock function belt)
Rear facing Infant seat/convertible seat
STEP 1 Place the child restraint system
on the rear seat facing the rear
of the vehicle.
STEP 2 Run the seat belt through the
child restraint system and insert
the plate into the buckle. Make
sure that the belt is not twisted.
STEP 3 Fully extend the shoulder belt
and allow it to retract to put it in
lock mode. In lock mode, the belt
cannot be extended.
STEP 4 While pushing the child restraint
system down into the rear seat,
allow the shoulder belt to retract
until the child restraint system is
securely in place.
After the shoulder belt has
retracted to a point where there is
no slack in the belt, pull the belt to
check that it cannot be extended.