What is wrong
You are right to be afraid. Somewhere along the way your theme was customised, and pressing update would hand you back a clean version of the theme with every one of those customisations gone.
So the update never happens, and the Store falls further behind on speed, on phones, and on everything Shopify has added since.
What it costs you
An old theme is slower on a phone than a current one, and phone speed is where small Stores lose orders they never hear about.
It also quietly blocks other work. Several of the fixes on this site are cheaper and safer on a theme that is close to current.
What we do
We go through your live theme and write down every customisation in it, one by one, before touching anything. Then we build the current version of your theme on a copy, put each of those customisations back, and hand you the two side by side so you can click through both before anything is published.
Your live Store keeps running the whole time. Publishing is your decision, on your day, and the old theme stays in your library afterwards.
Said before you buy, not after: this is the one piece we cannot price from your Store address. How much work it is depends on what previous hands left in the theme.
Price and time
Quoted per Store, from $1,250. The quote comes before any work starts, and it does not move afterwards.
We look at your theme and give you the number for free.