When using the SharePoint Update Item action in Microsoft Power Automate it forces you to enter values for mandatory columns. For lists with many columns this can be a long process if you only want to update a few fields. Example:

This list item contains a number of fields:
Field Name | Type | Mandatory |
Name | Single Line of Text | Yes |
Department | Single Line of Text | Yes |
Gender | Choice | No |
Marital Status | Choice | No |
Status | Multiple Selection Choice | No |
Homepage | Hyperlink or Picture | No |
If you want to be able to update one or more of those fields without supplying the information for every field, there is an easy way to do it.
The Update Item Trick
The update item action helpfully retrieves the column information from SharePoint for us to give us the fields to fill in. Most of the time, this is what we want. But here is how to force it so that it cannot do that.
Simply Create a Compose action and type the name of your list into it. Then create an Update Item action, choose your SharePoint site as normal, but for the list name choose Enter Custom Value and from the dynamic content panel choose the output of your compose action:

Now your Update Item action will not suggest any columns at all and you must supply this information manually:

Updating the SharePoint List Item
Our example list has a bunch of different column types and they are all updated in a slightly different way. So I will give examples of how to update each type, and then update multiple fields in a single request.
Single Line of Text
The single line of text field is the easiest to update. Simply supply the field name and the new value, like this:
{ "Title": "Paulie" }
Hyperlink
With the hyperlink we need to first supply the field name, and then the URL and description.
{ "homepage": { "Url": "https://www.tachytelic.net", "Description": "somesite" } }
Single Choice Column
The single choice column can only take a single value. So, only the field name and value are required.
{ "Marital_x0020_Status": { "Value": "Common-Law" } }
Multi Choice Column
The multi choice column must be supplied with an array of values, so an example would look like this:
{ "Status": [ { "Value": "Subscribed" }, { "Value": "Liked" } ] }
Updating Multiple Columns
If you want to update multiple values, you can easily combine the examples above, like this:
{ "Title": "Paulie", "homepage": { "Url": "https://www.tachytelic.net", "Description": "somesite" }, "Marital_x0020_Status": { "Value": "Common-Law" }, "Status": [{ "Value": "Subscribed" }, { "Value": "Liked" } ] }

Conclusion
So, as you can see from the above, by supplying the JSON directly to the Update Item action, you can easily update columns without having to supply the information for every other column again.
Hope this helps.