[ Folder A ] ---- index.html ---- [ Folder B ] -------- page.html
Assuming the above is your folder structure (as described) and you are going from page.html in Folder B to index.html in Folder A, then you don't need to explicitly specify the folder name again. The following should be sufficient
<!-- Link to Index File in Folder A(index.html) from page.html--> <a href="../index.html"> Home </a>
The ../index.html meant, leave current directory (Folder B) and go to the upper directory (Folder A) inside of which you would find index.html.
Note: A piece of advice is to use lowercase one word (or with underscore or hyphens instead of space) for your folder names in order to avoid issues with deploying on windows and linux system (e.g. foldera, folder-a, folder_a)
folder Bthen the path../folder B/folder A/index.htmlmeans "go up one folder (the ..) then go down one folder into 'folder B' then go down one more folder into 'folder A' — which is clearly not the folder structure you describe.