question goes to lenovo ThinkPad users...
option 1: It is possible to replace the hdd with a new ssd;

option 2: you can install a ssd extra to the existing hdd;
? im confused

Both, depending on the model.
Which series?

ebay -> laptop hdd to ssd -> buy -> screw that shit in -> fuck bitches -> enjoy your fast laptop

some of these steps are optional

Thinkwiki is a good source.
And of course the thinkpad forums, those guys build hybrids from mixed models, fix firmware issues themselves, generally they know more than IBM 😀
I don't know how many thinkpads I own 😛

@nbamaral X230 for example

@jakobev pretty sure it a straight forward replacement, sata3 ssd. Did that on a older x200. No extra disk unless you can use the second pcie slot. Not sure you have room for that.
Check https://forum.thinkpads.com/viewfor...
They are usually very helpful.

@nbamaral thanks for the source. im confused, cause in several videos i watched, they either just replaced the hdd on the side with ssd or opened the notebook and put the ssd under the keyboard..

I don't know x230 that well, but I'd say in that model, lifting the keyboard is the best way to access its guts. If you don't see a drawer on the side or a specific plastic top underneath (one screw, disk symbol) remove just the keyboard screws, and carefully slide it up to the screen direction, lift and disconnect it. Hope this helps :)

@nbamaral yeah for sure, thanks mate