I have probably posted something like this before.
So I have a document in which the first page is one column, pages 2 to 5 are 2 columns and pages 6 and 7 are one column. So I have tried all suggestions based on using section break, page break and column break.
What happens is page 6 changes pages 2 to 5 to one column.
So I need a step by step instruction on how to do this. It was easier in MS Word as each section could have its columns set.
I would appreciate some help.
Thanks
You must clearly separate the pages based on number of columns. This is done with page styles.
If page 1 is a title/cover page not related to pages 6 and 7, it is better to create a specific page style for it (otherwise a common one for 1 and 6-7 will do).
I suggest the following structure: Cover for page 1, TwoCols for pages 2-5, built-in Default Style for pp. 6-7.
To create Cover and TwoCols:
F11 to open the style side pane and click on the fourth icon in the toolbar (Page Styles)New
Page tabHeader and Footer tabColumns tabOrganizer tabFor Cover, additionally select TwoCols from the drop-down Next Style menu (which means TwoCols must be defined before Cover) so that you automatically switch from Cover to TwoCols on page break.
OK
Adjust Default Style properties (margins, header, footer) so that they are consistent with the other styles by right clicking on its name and Modify.
With these styles, begin page 1 by forcing it at Cover: with the cursor in the page, double-click on Cover page style in the side pane.
Note: this double-click is the standard way to “restyle” a page. It could be useful if you don’t retype your document. But, ponder on what you’ve done so far so as not to create a mess.
When you want to leave for page 2, insert a manual page break Insert>Manual Break, Page variant. It should switch to TwoCols but if you want to make sure, you can force TwoCols style.
Similarly, to switch to 1-column format, insert a page break Insert>Manual Break, Page variant, forcing style to Default Style.
If this answer helped you, please accept it by clicking the check mark 
to the left and, karma permitting, upvote it. If this resolves your problem, close the question, that will help other people with the same question.
It was easier in MS Word as each section could have its columns set.
You also could set Writer sections for you purposes. But be aware that Writer sections are not the same as Word sections.
Regard ajlittoz’s hints concerning page styles.
If you use Writer sections you don’t have to change headers/footers; they still remain because they are based on the same page style.
Little (!) help here: Section - LibreOffice Help
Better help here (page 137ff): https://wiki.documentfoundation.org/images/0/0d/WG54-WriterGuideLO.pdf
Download your sample file which includes some advice or comments.
Sections are better fit for short parts which do not disrupt the flow of discourse, like examples, quotations, explanations, illustrations. Pages with different number of columns highlight a stronger semantic break and result in logically more independent parts. Choice highly depend on your intent.
Remember also that switching from section to another is more tricky if you didn’t planned ahead where to put the bound.