There are three main use cases of attribute editing:
First, we might want to edit the attributes of a specific feature, for example, to fix a wrong name
Second, we might want to edit the attributes of a group of features
Third, we might want to change the attributes of all features within a layer
All three use cases are covered by the functionality available through the attribute table. We can access it by going to Layer | Open Attribute Table, using the Open Attribute Table button present in the Attributes toolbar, or in the layer name context menu.
To change an attribute value, we always have to enable editing first.
Then, we can double-click on any cell in the attribute table to activate the input mode, as shown in the upper dialog of the following screenshot, where I am editing
NAME_2of the first feature:
Pressing the Enter key confirms the change, but to save the new value permanently, we also have to click on the Save Edit(s) button...