You're not supposed to encrypt one character at a time! You need to turn an entire message into a single number, and then perform the modulo exponentiation on that plaintext number to get the ciphertext number. Then you do the exponentiation with the private decryption key to reverse the process from the cipher to the plaintext, and then turn that number back into the message.
Two optimizations speed up the process:
First, when you compute $m^e \bmod n$ you don't do it by computing $m^e$ then reducing it $\bmod n$. You do it by reducing $\bmod n$ after each multiplication in the computation of $m^e$.
Second, you don't work your way up from $m$ to $m^e$ by stepping through all the powers of $m$. You compute $m^2$ with one multiplication, then $m^4 = m^2 m^2$ with another multiplication, then $m^8 = m^4 m^4$, and so on, reaching $m^{2^k}$ after $k$ steps. Using the binary representation of $e$, you select a subset of $\{m, m^2, ..., m^{2^k}\}$ whose product is $m^e$. This is called the "square and multiply" method.
Every intermediate result in the computation is immediately reduced modulo $n$, and the only operation you perform is multiplication, so you never see a number bigger than $n^2$, which is way smaller than the unreduced $m^e$ when the numbers are of the size needed for practical cryptography.
Some programming languages contain a "modular exponentiation" function which takes the 3 arguments $m$, $e$, and $n$ and returns $m^e \bmod n$ using the above method. When working with a lower-level language that doesn't include it, you will write it first. (It's not hard if you already have the big integer multiplication taken care of.) Since I see this question originated on the mathematica site, here's the function in mathematica: PowerMod
I don't see a modexp button on your javascript calculator, so it's really not the right tool to use here (unless you want to work your way through the square and multiply method by hand - maybe that's good to do once to get the idea burned into your brain, but after that it'll be boring)
Best Answer
Multiplying by $256=2^8$ is equivalent to a left shift of $8$ bits. To invert, you basically unshift the bits out, so to speak.
To illustrate, suppose $i$ is the integer. Then, to recover $\text{char_n}$, just take $\text{char_n} = i \,\text{%}\, 256$. To recover $\text{char_n-1}$, do something like $\text{char_n-1} = (i \text{>>} 8) \,\text{%}\, 256$. The pattern should be clear from this.
($\,\text{%}\, $ is the $\mathbb{mod}$ operator, and $\text{>>}$ is the right shift operator.)