Hello, I hope the following assists you.The filter is a case of turning it about quarter of a turn to unlock it then pull it down to remove, to fit the new one is a case of lining the marks up, pushing the filter up then turning it until the lock marks line up. Below is a diagram to confirm this.
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The main problem you may have is to make sure you bleed the fuel system when the filter is replaced, don't run the engine until you have done so otherwise it will make it even harder to bleed. To bleed the system you need to fit an inline pump between the filter and fuel pump and force the fuel through, start by just connecting it to the filter outlet and pump it until diesel comes out then connect the other end to the pipe to the fuel pump[, pump the primer until it is stiff and crank the engine while an assistant continues to pump the primer. Once the engine starts allow it to idle for a few minutes with the hand primer still inline then turn it off and remove the primer.
Without bleeding the system you will find it nearly impossible to start the engine.
An example of the type of priming pump is at http://www.ebay.co.uk/itm/Draper-43986-Expert-Fuel-Line-Filter-Primer-Hand-Priming-Pump-Diesel-Engine-Tool-/221415483506?pt=UK_Hand_Tools_Equipment&hash=item338d642872
Let me know if I can assist further and I'll reply as soon as I'm able to.