Replacing a rainwater downpipe - how?

Hello everyone,

we are now homeowners and need to enlarge the connection of two rainwater downpipes into the ground from a 70mm diameter to a 100mm diameter.
Why? Because everything else (including the orange underground pipes) is 100mm, and the roofer, for reasons unknown to us, inserted a 70mm pipe in between during the previous owners' time. The result: the water backs up at the edge of the house because it cannot drain properly through the narrowing.

I have never done this before, but I want to first consider whether we can do it ourselves. 200 euros for materials and installation (if we expose the pipes) seems a bit much to me – but maybe it's normal?
Tomorrow we will also call the above-mentioned roofer again – after all, from our point of view, he made a mess of it.

Either way: what do we need to consider? How are two pipes connected so that it really, really holds/seals?

Best regards
Trilla

I've never done this before, but I want to first consider whether we can do it ourselves. 200 euros for material and installation (if we expose the pipes) seems a bit much to me - but maybe it's normal?

200€... honestly? With such an amount, I wouldn't even hesitate. When building a house, you usually only deal with costs in the four-digit range – it shouldn't be different for a renovation either.

A craftsman's hour costs, including taxes, probably 50-80€, depending on the trade... it probably can't get any cheaper than that.

It also depends on what kind of downpipe it is. There are some made of metal and some made of plastic. Some of the metal ones are soldered. As a layperson, you can't necessarily replace these without the appropriate equipment. However, there are also galvanized pipes, which are often designed to be connected by fitting together and you can do it yourself. I just bought a complete set consisting of a gutter and a downpipe, each 4 meters long, for under 100 € on eBay.