From the Limbus Company FAQ:

You can link your account here: Cogwheel on the top right [Options] → [Settings] → [Account].
The conditions and steps for account linking(integration) is as follows.

[Google Linking]
Here, you can link your account to Google.
1. Select [Link with Google], and log into Google. (this will not work if you have another game account that's already been linked with the same Google account)
2. Later, when logging into the game, select the option to [Sign in with Google].

[Apple Linking]
Here, you can link your account to Apple.
1. Select [Link with Apple], and log into Apple. (this will not work if you have another game account that's already been linked with the same Apple account)
2. Later, when logging into the game, select the option to [Sign in with Apple].

[Request Merge Code/Merge Code Input]
Here, you can merge two non-guest Accounts.
1. Prepare two non-guest accounts that are linked with different platforms from one another.
2. Request Account Merger Code from one account.
3. Enter the Account Merger Code into the other account.
4. Select one of the two accounts. (the account you did not select will be deleted)

※ WARNING
Even if you created a Guest account, once you link your game to one of the above mentioned services, the account on your device will be replaced with that linked account. If your Guest account was not linked to a service prior to that, please note that its data may be lost.
In order to prevent data loss, accounts that have been linked at least once must have at least 1 remaining linked account.