it is best to get the automatic transmission oil changed. your car probably uses dexron VI, but since toyota's own fluids are available, it would be better to get toyota ATF.
whether this solves the problem or not, you should get the transmission oil changed. in fact, it is recommend getting all the fluids changed and all the belts and connections checked when a used car is bought.
You can find Toyota ATF almost anywhere in karachi, but you'd prefer going to tibet center/rimpa plaza if you want the best price (trust me the difference is there)
For getting it changed, you can do it yourself if you want, it is very easy. if not, almost any mechanic can do it, visit the one you trust.
and yes, do update !