You'll have to give me a little more to go on than that!
The Lua library to manipulate strings is called, amazingly enough, the strings library.
https://www.lua.org/pil/20.html
If you have a numerical string and want to reliably use it in maths use tonumber() to convert it to a number type, if it isn't actually a valid number though that will just cause an error. In most simple cases Lua will quite happily do the conversions either way itself though.
For example if you have:
local a = 10
local b = "cat"
local c = b .. a
Then c will be the string "cat10"
Similarly:
local a = "5.0"
local b = 1
local c = a + b
will usually result in c being the number 1.5 but to be safer use:
local c = tonumber( a ) + b
If otoh you actually want a string result then you can do tostring( c ).
Been there, done that, got all the T-Shirts!
