Do bear in mind that the rollout for this version of the mobile operating system will happen in stages, so if you have not received yours yet on your handset, play the patience card as it might take a few more days before the entire slew of devices are on the receiving end of the similar update notification.
Upon installation, your software version should be bumped up to G900T3UVU1GPE1 or G900TUVU1GPE1, depending on the Galaxy S5 model that you happen to have in your possession. Regardless, the update should weigh in at approximately 1GB in size, so making use of Wi-Fi connectivity is highly recommended. Apart from that, you can always opt for Samsung Smart Switch for PC should you prefer to take the PC route when it comes to such an installation. [T-Mobile Support Docs]