Bought a jigsaw a bosch pst 54e how do i change the blade.thanks

Bosch jigsaws use a bayonet fitting blade that is retained by a single screw located inside the quill accessed from the top by either a long slim screwdriver or hexagon key. Sometimes a plastic cap must be removed for access to the top of the quill and some models have an in-built release mechanism.

Look at instructions that came with tool. Look on line for answer, It took me less than 10 seconds.
https://www.manualslib.com/manual/1091631/Bosch-Pst-700-Pe.html#manual

If it works like mine, you need a long, thin screw driver (flat) to tighten a screw that holds the blade. The screwdriver goes in, in line with the blade, on the opposite side.

With a long middle size screwdriver
in the hole at the top of the machine http://www.mtmc.co.uk/Bosch-PST-53-A-/-0603229203-Spare-Parts__p-88148.aspx

http://powertool.manualsonline.com/manuals/mfg/bosch_power_tools/bosch_power_tools_saw_product_list.html
hope it helps

there should be a allen head bolt in a slot at the top of the blade. you loosen it and pull out blade , replace blade and tighten bolt back up. hope this helps. have a good day,

There's an exploded diagram of a similar model here. Don't let it confuse you though. I mention it, because it shows the clamp screw which hold the blade in place (35 on the diagram).

Unplug the saw before you change the blade.

In the handle directly over the blade there is a hole with a clamp screw at the bottom. Use a long thin blade screw driver to loosen the clamp screw.

Once the clamp screw is loosened, twist the blade about 90 deg and you should be able to pull it out (don't cut yourself, by accident)

Put the new blade in with the same orientation and twist so the blades are facing the front of the saw.

Tighten up the clamp screw and you're good to go.

Be careful not to tip your saw over while the screw is loose. If it falls out and you lose it they're a bit of a nuisance to come by.

I couldn't find that specific model number Bosch saw, but here is the link to get user manuals from Bosch in case you mistyped it. (It's a long link that wouldn't all fit here, so I had to put a carriage return in it)
http://www.boschtools.com/Service/ProductServices/Pages/ManualsandPartsDefault.aspx
Or they may have a similar saw under a new model number. Here's the current model number of a very popular saw that has a less-than-obvious blade changing method 1587avsk

Hi just wondering what the screw looks like as I have an old one of these that the screw went missing out of years ago. I'd like to replace it but need to know length and diameter of the screw if you could please?