2.5 is already to the nearest tenth. 3 is the the nearest whole number.
Here's an example:
2.478382
The 4 is in the tenth spot, so you look at the next unit over, which in this example is the 7.
To round, the key is 4 and below, don't change, 5 and above, round up.
2.478382 would round to 2.5.
Here's another example just so it's clear:
For 3.7913, the 9 would make the 7 round up and the answer would be 3.8.
One unit after the decimal is the tenth place. Two units is the hundredth, three is the thousandth, and so on.
I hope that makes sense. You're good to go on getting 2.5 to the nearest tenth— it's already there.