s3 9300 to lollipop topic

Newbi here looking for some help.

I have a galaxy s3 i9300 and I want to go to lollipop. The flash counter was tripped twice on the phone by the store I bought the phone from, so it doesnt automatically update.

I dont care about losing any and all data on the device. Can someone point me to the simplest way to do this? I did search but getting lost with some of the terms and unsure what applies to me. The only thing I need to make sure of is the phone still has no issues when I use different sims when I switch carriers in different countries when traveling.

Thanks for the help