First of all:

Warning for Product Unit of Measure
The precision has been reduced for Product Unit of Measure.
Note that existing data WON'T be updated by this change.

As decimal precisions impact the whole system, this may cause critical issues.
E.g. reducing the precision could disturb your financial balance.

Therefore, changing decimal precisions in a running database is not recommended.

This means, while some Decimal Accuracy settings can be reverted, doing so may have unwanted side effects!

Thus, ensure this change actually works for you and does what you expect it to do in a test environment prior to changing your production environment (i.e.  in your test/staging environment, or at least in runbot.odoo.com)!

In any case, when the following is your situation:

You enter a Quantity of 0.236 and Odoo rounds it to 0.24 (or 0.240 in case you've increased the Decimal Accuracy* already)

... and instead you want it to actually reflect your input of 0.236:

*) Setting/increasing Decimal Accuracy:

Enable Developer Mode (see https://www.odoo.com/documentation/18.0/applications/general/developer_mode.html), then navigate to Settings -> Technical -> Database Structure --> Decimal Accuracy. From the list, find the record Product Unit of Measure and change Digits to 3 (note: trying to revert this back to a lower accuracy is what will return initially mentioned warning!). This will result in an output of 0.240 instead of 0.24.

Use higher precision for Units of Measure:

Then, in order to actually use a higher precision, navigate to Settings again and search for 'Units of Measure' (depending on the Apps installed this may return multiple results):

Here, click the Units of Measure link and open the Units of Measure Category of the Unit of Measure that needs a higher precision, i.e. 'Unit' in above example, and set the Rounding Precision accordingly (here, change it from 0.01 to 0.001):

Once saved, a new POS session (close and reopen the POS session) should allow you to use higher precision on Products using this Unit of Measure.