How do I update to the latest version of Catalina 10.15.7, without updating to Big Sur?

Hello

My mother has a 2017 Macbook Pro. She is currently running macOS Catalina 10.14 and she hasn't updated het software since.

She wants to install iMovie (I deleted it when she purchased the macbook a year ago, but now she has taken an interest), but the app store prompts her that she needs 10.15.6 or higher.

How do I update to the latest version of Catalina, without updating to Big Sur? She currently gets prompted to install Big Sur, but it's been a mess on my Macbook Pro and I don't want her to have the same issues.