The easiest way to migrate from a legacy Safe mobile app to Gnosis Safe would be creating a new Safe and simply transferring funds over. However, if you want to preserve your Gnosis Safe account and just modify it so it is possible to use it with the Gnosis Safe interface, this is a possibility, too.
As a prerequisite, you need to set up your web interface to work with a Safe originally created on mobile as described in this help center article.
Warning: After executing the following steps you will not be able to use you Safe with the legacy mobile app anymore.
Please check the currently set threshold via Settings -> Policies.
If it says “1 out of 3”, we recommend going to Settings -> Owners and remove all owners except the currently connected Metamask.
If it says “2 out of 4”, we recommend removing all owners except the 2 Metamask accounts that you have set up as owners of you Safe.
Now you are able to configure your Safe setup as you prefer. We recommend considering the following:
Use hardware wallets such as Ledger or Trezor as owners.
Use a threshold >= 2
For threshold n add at least n + 1 owners. This allows you to recover in case 1 owner cannot access their account anymore.