I have an older version of Microsoft Office and now I have to update it. I only need Microsoft Word. Does anyone know what it costs to upgrade Word?

Look into the Free LibreOffice, it handles MS Office files…

https://www.libreoffice.org/

Apple macOS (Mac OS X)

The software and hardware prerequisites for installing on a Apple macOS computer are as follows:

• LibreOffice 6.3.x: macOS 10.10 (Yosemite) or higher
• LibreOffice 6.2.x: macOS 10.9 (Mavericks) or higher
• Intel processor
• 512 MB RAM
• Up to 800 MB available hard disk space
• 1024x768 graphic device with 256 colors (higher resolution recommended)

It is a recommended best practice to back-up your system and data before you remove or install software.

Notes:

• For certain features of the software - but not most - Java is required. Java is notably required for Base.
• Due to an issue, there are the following restrictions with Java: on macOS 10.10 and newer, JRE isn't found, JDK is required
• LibreOffice 4.3 can still run on OS X 10.6+, but please understand that this support ended when 4.3 was retired on May 27, 2015.

https://www.libreoffice.org/get-help/system-requirements/#Apple

Feature Comparison of LibreOffice v6.4 with Office 365/Office 2019 for Mac

https://wiki.documentfoundation.org/Feature_Comparison:_LibreOffice_-_Microsoft_Office

It is possible to purchase only Word, but is so grossly overpriced you may as well just get the suite.

Word, all on its own, is $139. For $150 you can get Office 2019 and get Word, Excel and PowerPoint. Either choice is a one-time cost perpetual license. It just isn't worth saving $10 to gyp yourself out of the other two titles.

Buy Microsoft Office