Depending on your project this can get complex and even expensive if you used a lot of 3rd-party libraries.
You can find a good overview here. If you google for "migrating delphi projects" the first hits (for me) list a number of youtube videos that may be useful. There is also a fairly recent book on the topic available: "Delphi Legacy Projects: Strategies and Survival Guide" from William H Meyer.
Do not try to open the old project file directly, first delete all files with extensions other than dpr, pas, inc, rc, res from the project directory (after making a backup, of course) and then open the project's dpr file in the 10.3 IDE. That creates a new project (dproj) file. Open the project options dialog and adjust any pathes in the compiler page as required. Make sure to enable all hints and warnings and then try to build the project. The hints and warnings you get even if the build is successful (which is unlikely) will give you an inkling about the magnitude of the task you are facing.
The main pain points are:
Ansi to Unicode conversions.
3rd-party components used (i hope you have source code for all of them). Even if D 10 Versions exist for the old components (which you have to buy if commercial) there are likely to be a lot more compatibility issues with them compared to Delphi RTL or VCL code.
Database access if you used the BDE (which is dead, buried and cremated for good measure).
Assuming sizeof(pointer) = sizeof(integer) (especially if you want to go 64 bit) or sizeof(char) = sizeof(byte) when misusing strings as storage for binary data.
Good luck