As far as I know ExpressionEngine will use one or the other, SHA1 or MD5 (I believe SHA1 is the default). In my test it actually worked to use either one when replacing the password, allowing me to login just fine in both cases.
You can use MySQL to generate a new password hash for yourself. For example either
SELECT SHA1('pass123') or
SELECT MD5('pass123') will generate a string for you.
The following MySQL query would set your new password to "pass123" (where your
member_id is 1):
SET password = SHA1('pass123')
WHERE member_id = 1
Or if you don't want to use MySQL then this online SHA1 hash generator or MD5 hash generator tool also works. Just copy the string and then using PHPMyAdmin, Sequel Pro, or similar, find your user record in the
exp_members table and replace your current
password with the new string .