First this is PC not console or XBox. But with your battle count it seems you play both.
Reason I mention this is it may be different on console as I don't play that platform. But I don't think (key word) it would be different except maybe in the menus.
There's two things that cover what you mention, so I'll cover both.
On tanks that are elited meaning you have everything researched including any tanks that come off that tank you have the option to turn on accelerated crew training by checking a box above the crew.
This will train the least trained crew member (they're flagged different than the rest of the crew in PC) at essentially twice the rate of the other members because the xp that would normally be applied to the tank is used on them. There is no cost to do this and it also is a option on premium tanks.
The other thing is you can spend free xp you have accumulated to boost the training of any crew member. This burns free xp which you accumulate a little every battle, but it's mostly obtained from converting xp on elited tanks with gold. Personally because of the way xp scales this isn't a very good "bang for your buck" even if you use free xp you received without using gold to convert.
This is done on each individual crew members personnel file. Right click the crew member then personnel file. Then in the lower left there will be a box to click to increase their training level using free xp. You can check the amounts without using any because it takes a confirmation click to complete the process. At least this is how it's done on PC.
Hope that helps if not post again or feel free to PM me.
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