Hopefully you didn't find the game too easy, because it's actually impossible to change the difficulty to a higher version. You either have to stick with the difficulty you chose, or begin a new save on a more difficult difficulty.
However, if you want to make it easier, then you are able to!
Here's how to change the difficulty in Resident Evil 2 if you want to make it easierThe method to change the difficulty to be easier in Resident Evil 2 is to die a whole lot of times! After a certain amount of deaths, the game will ask you if you want to lower the difficulty during the "Game Over" screen. If you play on hardcore, it will ask you if you want to lower it to Standard, and it will ask you to lower it to Assisted if you die a lot on a Standard difficulty game file. You can go from Hardcore to Assisted as well, but you have to first lower it to Standard, then die many times again until the game ask you once more to lower the difficulty.
There is no fixed number before the option pop up, but it should not take too long if you die repeatedly to get the option.
As mentioned above, it is impossible to increase the difficulty of the game, so this is a permanent choice. You can't lower it for a while and then turn it up again later, but you must instead create a new save file if you want to make it more difficult.