this tutorial is for windows, sorry if you're mac or linux but i have windows lolz
i'd firstly suggest checking out
this page, specifically the
edit a save section. before doing this, please make sure to backup your save files!
install sublime text and the two packages that the wiki recommends, it's really easy so if you have questions just ask i can help you further
anyway, in your save files folder (%appdata%\StardewValley\Saves) open up the file [YOUR FARM NAME]_[NUMBERS] (for example mine is Lamare_832525 or whatever tf the numbers are, don't open up the one that has _old at the end) and open up the file that says SaveGameInfo in sublime text.
in the first file I told you to open, use ctrl+f to find <NPC xsi:type="Dog"> and change the dog to Cat.
next, go to the second file i told you to open (savegameinfo) and find "<dogPerson>true</dogPerson>" and change the dog to cat. you can change the breed number here but idk what the breed numbers are, maybe play around with it. The name should be the same and your friendship with your pet will carry through as well
let me know if this didn't work for you, and, again, make sure to back up your save before trying this out ^^ good luck