From 8gb to 16gb
If you need an 8 gb bar that's around 30 euro I think 16 gb is twice as much
An additional 8 GB bar should cost something around 30-35 euro. Just make sure you take SO-DIMM, the others are for desktop PCs
Depends entirely on your laptop. Do you want to do it yourself? Then it only costs as much as one module.
If you run out of slots, then you have to replace the RAM. So if you have installed 2 * 4GB, you have to install 2 * 8GB. If you still have a slot free, you can insert 1 * 8GB.
An 8GB bar costs approx. 30 euro
https://geizhals.de/...at&hloc=de
A 16GB kit costs just over 50 euro
https://geizhals.de/...at&hloc=de
If you don't want to upgrade yourself, you have to ask a computer shop nearby.
But also make sure that you really have DDR4 RAM, otherwise you would have to resort to older DDR3
