Where to start learning disassembly and reverse engineering?

Question

reverse engineering

At work, I came across the fact that there is a software package that is in active operation,
but there are no source codes and there is no longer a company that made this complex.
It is necessary to correct errors and make changes to the algorithms of the complex.
Writing a new software package is a huge amount of work.
Most of the specialists who work with this software package know where to go and what to click,
but at the same time I don’t understand how to do with their hands what is implemented in the complex.

Answer 1

Stay away from this venture. There are business owners - so they decide whether to write new or break the old. You just figure out what will happen if you screw up by "correcting mistakes and making changes."
In general, there is no significant market for "disassembly and reverse engineering", and these skills will not add career potential to you.
And if it's so hot that you just don't have the strength, learn to write in assembler, without it - nothing.

Answer 2

Addendum to the @longclaps answer
Assembler
+ device operating systems in general
+ device specifically for the one for which the complex is written
+ if the complex uses drivers, skills in writing and reversing drivers
+ the ability to use special programs such as Olly, IDA, Gidra, etc. (respectively, knowledge is needed from all the previous points)
+ knowledge of some Python for automation
Sometimes, in order to restore the algorithm, you need to know how compilers digest sources. Because what the programmer writes and what is then obtained at the output is not always the same :)
The main thing is not to take the initiative, otherwise you risk making a big lump of problems. As already mentioned, it's up to the owners to decide, not you.
The reverse is worth downloading if this area attracts and you want to work in IB offices like Casper. And since the market is very small and there is less money, it’s more difficult to roll in.
*(compared to development)