You can't, as there is no reset code available once the door is locked. It would be absolutely useless anyway if all a thief had to do was to ask on a public website for a bypass code. The only alternative way into the safe is via the override key (an actual physical key which fits into an opening hidden behind a removable plastic panel on the safe door).
If you cannot find the emergency key then you have just one fix: you must open the safe destructively. Fortunately, your model is just a budget-priced electronic strong-box rather than a true safe, so the top or sides can be easily be cut open using a grinding disc or by just drilling a few holes and joining them up with a cold chisel.
Note that it's not worth calling out a locksmith, they'll only cut the box open and the charge will be many time more than the cost of a complete replacement strong-box (you can buy a similar complete replacement for under £40 including postage from this seller