To flash your phone you can use setool2lite v1.11 or XS++ v3.1 if its CID is 52 or less. If it is CID 53 you must use Far Manager + JD Flasher.

There are many tutorials and guides for the three tools.

You can flash K800i into K810i (never into K850i). You will need Main, FS and CDA.

You can not request files which are in downloads section. Please buy a premium account or make your way through the forum to gain access to them.

Use search option for the files you are trying to find. A lot is posted here, and if you do not find it, I will search for it in the Spanish forum.