Hello Nancy, Ben here - your Shopify Guru!
Good news! This is very easy to do! Every theme has language settings, these are stored values that are used throughout the theme.
To access your theme's languange options from your admin in the left menu select Online store then Themes then do the following:
You will then be in your language settings, at the top you will have tabs, click on Collections.
You can then change the Catalog title to be Collections.
I hope this helps! Feel free to reach out if you have any further questions :)
Ben C - Shopify Guru