Changing data in multiple cells at once?

Hello, I received a spreadsheet with over a thousand item's info and in each column [L] there are numbers 20, 30, 45, 60 mixed and not all grouped, for example, they look like this:

30
30
20
45
30
20
45

and so on. Instead of having to change the info in each cell so 1,000 times I am seeking a way to do this so it will change all identical numbers with the same info, using the above example of numbers, I would like to change every cell with the number 30 to the number 40 and every cell with the number 45 to the number 55 and so on, is this possible please?

Yes, that works, thank you very much!

another solution.

CNTRL + H

seek 30

replace by 40

Change all