How to Increase the time span on a certain era??

I know there is some way you can change/do this without Mods. I've heard of something like going into files in the Civilization folder on your computer, and changing some numbers from 180 to 180000? It supposedly increases the time spent on a certain Era. (Medieval would be amazing) The problem, I don't know how to do it? I really need to find out how to do this with good instructions. And to be honest, I had this question for years and never knew how to increase the time . So please, if someone has an answer, post the reply here.

EDIT: And if there is such a Mod that Increases the time in an Era (Or even better, stops it at a certain Era) Then I'm gonna need to know how to download it WITHOUT Steam. Surely there's an alternative way to download a Mod on civ without doing the "Get Mods" button ingame that only connects to steam. Or maybe it doesn't Only connect to steam? Anyways, I Need a deeper explination on that too...

Thanks guys!

The span of an era is not turns. It ends for you when you know a technology of the next era or all the techs of this era. You can enlarge this only by adding a tech to the tech tree or by increasing the costs of techs.

What do you actually want?

Well, I wanted to either stop at a certain era, so further eras are locked and you can only go to, say, medieval era. If theres no way to completely stop an era from proceeding to the next era, then how can you increase the cost of an era? (not increase the turns on ALL the eras, like an option in-game has, but like for only 1 era/all the technologies it has.)
Example: I want to play a game where as soon as I reach the medieval era, it takes overall 180,000 turns to proceed to the next era. (Making each technology be 18,000 turns. Unlike the other before eras where it took only 50 turns. Highest 70 I think)
Ancient = 50 turns for a technology
Classical = 50 turns for a technology
Medieval = 10,000 turns for a technology

Is this possible? Theres a way to do it through the civilization files or something...

Hmmm. You should update the beaker cost of the techs from that era. It is one fix, although another fix would be to comprehensively change all science modifiers from every resource in late classical and all medieval.

The next question is that, you definitely should increase the beaker costs of all the later eras too, so that they 'seem' to flow at comparatively the same rate. It still won't be a simple formula there, because science is advancing during gameplay from more population and conquering cities. It would depend on how steep the function is.

You can achieve a smooth effect only with smoothness proportional to the effort you put in.

The manner of making these changes is either to produce a mod, which will update the representation of these data in the game at the time it loads, and which you can port to other users to have the same play experience as you, or to change the files in the game folder from which the game loads the data and configures itself.
The technology costs are in... Gameplay/XML/Technologies, in the table 'Technologies'.

I cannot figure out what function this is approximating: The tech costs are

35 55 105 175 275 | 485 780 1150 1600 | 2350 3100 4100 5100 6400 7700 8800 9500 (Future Tech)

Ancient - Medieval | Medieval - Industrial | Industrial -> Future

The differences are factors of these:
0, 2, 5, 7, 10, | 21, 29.5 37 45 | 75 75 100 100 130 130 110 70

The first part of the Medieval Era is a progression from the Classical, but then there's a huge jump in cost. Same for Industrial. But it's too irregular for me to be sure it's quadratic or something else. I'd have to use Serious Math, and I ain't got time fo dat.
You have to figure out the function so that you can know how to alter the rest of the tech costs appropriately. That is, you should compose the tech function with a function of your choosing on the medieval and later eras, rather than just multiply by something.