GIS software isn't really designed for adding custom text annotations that have no connection to the source data attributes.
If you wanted to have totally custom labels for each feature that should be stored in a separate field in your layer (e.g.
alt_label as J.R. suggested in the comments).
However some of these other options might help accomplish similar outcomes in some situations:
Label text substitutes
You can find this under Labels > Text tab in QGIS 3.x up to 3.8, Labels > Formatting in Q 3.10 onwards. You can import and export lists of substitutions as well to use in various projects.
Using expressions in your text label value
Instead of just using a field for your label you can use an expression and add custom rules to add/replace text for a specific feature or class of features as you see fit. Such as
case when "id" < 20000 then 'Not important' else to_string("id")||' (Important!) end
case when "id" = 34565 then 'Special Thing' else "name" end
and so on.