Are the strings still shiny & silver?  Are there any grey or black (nickel oxide) places on them?  Do they feel smooth or rough?  These are some of the things to look for when deciding to change the strings.  You also need to make sure the strings are wound tightly around the tuner posts so they don't slip out of tune easily when playing.

It seems odd that the E string went to G.  That either means the string went way dow in pitch (maybe the windings at the tuner post or the bridge got loose).  Or the string went up in pitch which seems very unlikely.

This tutorial covers all those possible problem areas:  what to look for in string life, when & how to change the strings, how to do it effectively to make sure your guitar stays in tune.

https://www.guitartricks.com/lesson/24112

If you don't want to deal with any of that, then take it back to the shop & pay them to check the guitar & put a new set of strings on it.  Hope that helps!