There isn't a super nice way to do this in Lua, unfortunately.

Here are two options. One based on processing the number as a number, the other by processing the number as a string.

Math

We can use modulus to find the thousandths+ part of the number, and remove that. If we want to round rather than truncate, we would add five thousandths to the number before doing this:

function truncate(x, places)
	return x - x % 10^(-places)
end

function round(x, places)
	return truncate(x + 10 ^ (-places) / 2, places)
end

Strings

We can turn the number into a string, then chop off the places too many after the decimal point. This can't (easily), only truncate.

x = tostring(1.99)
x = x:gsub("%.(%d%d)%d+",".%1");
-- a decimal point, followed by two digits, followed by at least on digit
-- becomes a decimal place followed by the first two digits

A cool property of the second one is that you can use it on full segments of text, not just the number itself.