This depends on the way you run OBS Studio. For example, If you are using shortcut (desktop icon) to start the application then you can adjust path to the obs64.exe file and folder (currently it is set to obs32.exe in your system) in shortcut properties. This can be done in two ways:
1) Right click on desktop icon of obs and click Properties. Then type manually "64bit" instead of "32bit" at Target and Start in fields, click OK button to save changes.
2) There is Find option in Windows file Explorer. Hit F3 key on the desktop, enter obs64.exe. From the list of search results choose obs64.exe that has obs icon - this is required file and path to it, remember it. Now you can adjust desktop shortcut to OBS Studio, right click on the file you just found and choose Send to>Desktop (create shortcut). Now you have new shortcut on your desktop that points to 64 bit (obs64.exe). Run it.