Replacing sealed unit in wooden frame

[Djnikon]

Hi all,
I have not long moved house and we have one DG unit that has blown, it's filthy inside and mists up every day without fail.
The unit is 600mm high by 400mm wide (so not huge) and has a putty bead inside and out (although the external bead has seen better days).
I intend to replace this in the coming week and looking for advice on how to best do this.
I have only ever changed a single glazing panel in a very old house before so not entirely without basic skills just a lack of indepth knowlegde .
Any advice is greatly appreciated as the wife want's this done sooner rather than later you guys would be helping to preserve my sanity and quiet life

 

[masona]

I'm assuming it's a timber because of the putty, if so, a flat sharp chisel to prise off the longest beading first but depending if the smaller beading will stop this because of the overlap of the beading, if so, then remove the smaller beading first. Failing that carefully punch the nails in with a very thin nail punch, the most important when you fit the d/g unit is the packing supporting the inner and outer glass on the bottom hence why most of the unit failed because of this but not always. If the packing not supporting both glass panel then over time the glass will start slipping causing the seal to break for mist problem

 

[Djnikon]

Hi Masona, yes it's a timber frame.
So when I remove the old unit, I need to pack the new one (with what and how high) to keep it off the bottom of the frame?
I also heard about things called frame splines (which are apparently triangle shaped with a folded head that push in to the frame with a bar rather than a hammer and nail/pin) but can't find any detail on such a product - has anyone heard of these?
Does the new unit need to be sealed with putty or can I use a silicon or mastik type solution?

Thanks again

 

[foxhole]

The new unit will come with the required packers, just small sections of dense foam which hold the glass out of any sitting water.