So, I can tell you what Etrade allows since that is what I use. Not sure if it applies to all brokers. Here is how it works for me:
If I had a bunch of lots of a particular ticker and wanted to sell the last lot I purchased (newest) but didn't pick lot selection and they took FIFO instead, I would have to go into my portfolio on the website. Choose gains/losses, use the filter and type in the ticker and the date of the trade. Now they only allow you to change the lot selection prior to settlement. So any sales that were executed before the two day period wouldn't be allowed to be changed. If you were to want the change on a stock purchased yesterday, you would make sure the dates are set to that particular date and then you have to click on the ticker to expand it. If you are within the dates where they allow it, you will see the "swap" selector and you can change it.
What you need to do for the future is change your selection mode. I have mine set so that it allows me to pick whichever lot I want. If I don't pick that mode it defaults to FIFO (first in, first out).
In terms of wash sales, you can still buy a stock within the 30 day period of a loss sale. So if you sold a stock and took a dollar loss per share on the sale, but then bought it back, that original loss will be deferred until you sell the next one and they will add that extra dollar that you lost to the new cost basis - making the new cost basis higher than what you bought it at.
