How do i fit a blade on a bosch pst 52a - Audiovox Bosch 718 World Cellular Phone

Bosch use bayonet fitting blades and once the retaining pressure is released the blade will turn a quarter turn and then can be withdrawn.

The retaining pressure is released by using a long thin screwdriver or allen key (depending on version) through the hole in the handle to access the screw in the top of the quill. Once engaged the screwdriver or allen key is turned anticlockwise.

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

Like this. https://www.manualslib.com/manual/1091631/Bosch-Pst-700-Pe.html?page=3#manual

not sure if this link will help try it.. Bosch pst 700 PE Original Instructions Manual Page 25 .
or this link.. Bosch PST 700 PAE manuals Google Search

On the top above the saw there is a hole where you can loose the blade with a screwdriver
http://www.powertools-aftersalesservice.com/public/boschprof/service?p_auth=ey3nVf9O&p_p_auth=U0WzCDpR&p_p_id=WSRP_bc93764e__6873__4b00__9a29__b331c9759da9&p_p_lifecycle=1&p_p_state=normal&p_p_mode=view&_WSRP_bc93764e__6873__4b00__9a29__b331c9759da9_wsrp-navigationalState=L0JPUy9QVEFTLVBULVNJUy1TaXRlL25sX05MLy0vRVVSL1ZpZXdQcm9kdWN0LURpc3BhdGNoO3BnaWQ9NGFmNWU1ODJiOWI1ZThkMTI2OTZlNzUwZmI5MmhiSFp1WnM0O3NpZD1hQWQxSl93STBEZHEycVM0NzRacDNQT0Rmc3pHeFRVVDhnYjJqSDZvb1J6dkp1ckQyLVU9

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

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