Heres how to upgrade tools via the save file code, up to iron

[nonamenoprob]

https://pastebin.com/pfRy3QAa

This person (NOT me!) pasted a string of code that will replace your tools with upgraded ones. They only did up to steel, unfortunately, but hey, if you want an easy upgrade from a lower tool then this is for you!

Just open your files, show hidden items if necessary,

Appdata>Roaming>StardewValley>Saves>[your save file here]

Before anything else copy the entire file, and make a back up somewhere safe.

open your file, and there will be one notepad document with your characters name, and one with the name SaveGameInfo, and possibly two more of each with '_old' at the end signifying a back up of the previous day. ignore the previous day files.

Go into SaveGameInfo, find the beginning of the string for the tool you want to replace with an upgraded one. Say it's the Axe.

Find
<Item xsi:type="Axe"><specialVariable........(more code).........numAttachmentSlots></Item>
It always starts with one '<Item', and ends with '<Item>'

So you know it's ended if you see two '<Item><Item's, because between those two is where one item ends and one begins. So just cut it off after the one, between the '><'

So copy the code for your new axe off of that link at the top, then delete your current string for axe out of your save game code. Then just paste the new axe in the same spot and save.

NOW do those same steps in your file named after your character so the two types of save files will match.

When finished, open your game and see if it worked.
Worked for me!!! Hope this helps someone else!!!