I have old version of Quicken, with file dating back to 2006. I want to delete several years posts

I assume by "posts" you actually mean several years of transactions in your Q data file.  Is that correct?

And, if so, why?  You won't get any speed/performance improvement by doing this and "putting Humpty together again" if/when you change your mind is almost impossible.

Further note that there are other actions that you can take to improve Q's speed.