The short answer to the titular question is that Jed Hoyer can upgrade the Cubs’ roster by signing or trading for better players. However, the reality of the situation is that there are 29 other teams vying for the services of those same players. We also have to consider the fact that for all their flaws, the Cubs really only have three positions in the field up for grabs.
We can stretch that to four by including a middling rotation that isn’t built to shut opponents down on a consistent basis, especially if Hoyer ends up running it back next season.