Unfortunately you’ve hit a very complicated area. Office 2013 encrypts files with AES when you password protect them, and this is a fairly secure algorithm to date. The only way to decrypt it is to have the key (password).
This means that the only solution left for you is to guess your way in. Either on your own, or with software that can brute force it. Office 2013 has a more complex implementation of encryption though, which makes brute forcing very slow.
If you choose the software route, please proceed with great caution. Much of the software advertised as a tool to decrypt or crack Excel files will be fake and could contain malware. I’d recommend doing some extensive research first. I do know there are some paid and reliable tools out there which I’ve seen used before.
Hope you get it sorted out quickly!