Is it fine to change th order of authors in Arxiv?
Modifying an arXiv preprint is done with the "Replace" feature, and this allows you to make arbitrary changes to the Author metadata field.  

There is no need to delete the paper, and trying to do so would likely cause more problems.  (You cannot actually delete an arXiv paper, you can only "withdraw" it, and trying to upload a preprint which is substantially identical to a withdrawn paper may be rejected due to excessive overlap.)

Just make sure that you also change the author order in the paper itself and upload new tex/pdf files, otherwise people will be confused.  And, of course, all of the authors must have consented to the new ordering.