You can edit your save file. Go to %appdata%/StardewValley/Saves and find the folder _ .

First, back up your save in case something goes wrong.

Go in there and use notepad to open the file with the same name as the folder. Ctrl+f for "experiencepoints". You'll see a series of 6 values like xy ... set all the numbers to 0. Then ctrl+f for "farminglevel". You'll see similar tags like: 10 ... set all those to 0. Also set the tag to 0. Assuming you didn't already get to level 5 or 10 and get any professions, this should reset you back to the beginning. If you did get professions, ctrl+f for professions and replace the tag with . Also replace the tag with if there was anything there.