The first word that comes to my mind is "Edit".
english.stackexchange.com/users/380/Mr. Shiny and New 安宇
Taken as a group, these are all examples of "Text Modification".
If I'm reading your comment to morganpdx correctly, you could make a title like "Text Modification Options" where you then give further information about "Appending to your text", "Prepending to your text" (or "Inserting before your text"), and "Replacing your text".
english.stackexchange.com/users/2490/Hellion
You already have the answer.
Prepend, append and replace are all verbs.
See below, with related nouns:
Append:
You append text after some other text.
I appended some text.
He will append some text.
Prepend:
You prepend text before some other text.
I prepended some text.
He will prepend some text.
In either case, both of them could be called a supplement, or supplemental text.
english.stackexchange.com/users/1267/morganpdx
You can call it conjugation too.
[My bad, I didn't notice the word "replace.
" You probably need to call it a "modification" already.]
english.stackexchange.com/users/2955/KahWee Teng
Just another suggestion.
In my experience, designers of systems often want to be systematic, which is to be expected, and want to catch similar things under a single heading.
But why not describe a thing by two terms?
Some Loeb translations of Plato translate many a complicated Greek word into two English words, which works rather well.
You could say "Replace or add to text" , or "Replace/Add" if you want it shorter.
english.stackexchange.com/users/3036/Cerberus_Reinstate_Monica
Let's assume I have a text in the notepad: "Some Text".  If I enter more words after the text it will be an append ("Some Text Another Text"), otherwise it will be a prepend ("Another Text Some Text"), if I don't replace the text with a new text completely ("Another Text").  So I am trying to come up with a term for these actions. How do you call it?
I am trying to come up with a term for actions Replace/Prepend/Append
english.stackexchange.com
<terminology><hypernyms>
english.stackexchange.com/questions/7702
english.stackexchange.com/users/3142/Ozgur
There are many different terms that you might use to replace the actions "Replace", "Prepend" and "Append", but some potential solutions include "Edit" or "Text Modification". Some suggest using  "edit". Some suggest this is actually  "Text Modification" and the words being used are correct. It could be called a conjugation. One option is "Replace or add to text"
There are many different terms that you might use to replace the actions "Replace", "Prepend" and "Append", but some potential solutions include "Edit" or "Text Modification". Simply "Edit". However, a more complete suggestion could be "Text Modification Options" followed by further information about "Appending to your text", "Prepending to your text" (or "Inserting before your text"), and "Replacing your text".
Suggestions include "Edit"; Text Modification"; or "Replace/Add to Text". There is no agreement as to how these actions would be called collectively, but some suggestions might be "Edit", "Text Modification" or simply "Replace/Add".
Some suggest "edit" This is "Text Modification" The words you are using are correct. It could be called a conjugation. One option is "Replace or add to text"
Edit. Text Modification or Text Modification Options where you then give further information about "Appending to your text", "Prepending to your text" (or "Inserting before your text"), and "Replacing your text".
There is no agreement as to how these actions would be called collectively, but some suggestions might be "Edit", "Text Modification" or simply "Replace/Add".